Since I have a bunch of Intel ATX motherboards in my possession, I've decided to compare them directly. It would be good reference point, because they all were made by Intel.
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Motherboards:
Intel TC430HX with 256 Kb L2 cache
Intel TC430HX with 512 Kb L2 cache
Intel LT430TX with 512 Kb L2 cache

More info about Intel motherboards: Intel-built 486/Pentium/PPro/PII motherboard guide

CPU:
Pentium MMX 200 Mhz

RAM:
64 Mb EDO / 64 Mb SDRAM CL2

Video card:
Matrox Mystique 220 4 Mb / GeForce 2 MX PCI 32 Mb (ASUS V7100Pro) / Voodoo 3 3000 PCI 16 Mb

Sound card:
None / Disabled

OS:
Windows 98 SE + DOS mode (BootGUI = 0)
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Software & Direct3D tests (Matrox):

Intel 430HX / L2 cache 256kb / Matrox Mystique

Quake v1.06 (DOS / VGA / Demo1) - 47.3 fps
Quake 2 v3.20 (Software / 320x240 / Demo01) - 22.4 fps
PC Player Direct3D Benchmark 2.10 (Software / 320x240) - 7.8 fps
PC Player Direct3D Benchmark 2.10 (Direct3D / 320x240) - 26.7 fps
Forsaken (Software / 320x240 / Ship Demo 400% speed) - 22.28 fps
Forsaken (Direct3D / 320x240 / Ship Demo 400% speed) - 52.59 fps
Incoming (Direct3D / 320x240 / Timedemo 1) - 21.28 fps

Intel 430HX / L2 cache 512kb / Matrox Mystique

Quake v1.06 (DOS / VGA / Demo1) - 49.0 fps
Quake 2 v3.20 (Software / 320x240 / Demo01) - 23.0 fps
PC Player Direct3D Benchmark 2.10 (Software / 320x240) - 7.9 fps
PC Player Direct3D Benchmark 2.10 (Direct3D / 320x240) - 27.1 fps
Forsaken (Software / 320x240 / Ship Demo 400% speed) - 23.16 fps
Forsaken (Direct3D / 320x240 / Ship Demo 400% speed) - 54.11 fps
Incoming (Direct3D / 320x240 / Timedemo 1) - 21.57 fps

Intel 430TX / L2 cache 512kb / Matrox Mystique

Quake v1.06 (DOS / VGA / Demo1) - 49.7 fps
Quake 2 v3.20 (Software / 320x240 / Demo01) - 23.3 fps
PC Player Direct3D Benchmark 2.10 (Software / 320x240) - 7.9 fps
PC Player Direct3D Benchmark 2.10 (Direct3D / 320x240) - 32.0 fps
Forsaken (Software / 320x240 / Ship Demo 400% speed) - 23.30 fps
Forsaken (Direct3D / 320x240 / Ship Demo 400% speed) - 55.87 fps
Incoming (Direct3D / 320x240 / Timedemo 1) - 23.73 fps

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Direct3D & OpenGL tests (Nvidia):

Intel 430HX / L2 cache 256kb / GeForce 2 MX PCI

Quake 2 v3.20 (OpenGL / 640x480 / Demo01) - 33.3 fps
PC Player Direct3D Benchmark 2.10 (Direct3D / 640x480) - 34.5 fps
Forsaken (Direct3D / 640x480 / Ship Demo 400% speed) - 74.92 fps
Incoming (Direct3D / 1024x768 / Timedemo 1) - 27.34 fps
Expendable (Direct3D / 640x480 / go.exe timedemo ) - 16.8 fps
MDK2 Demo (OpenGL / 640x480 / timedemo ) - 20.23 fps

Intel 430HX / L2 cache 512kb / GeForce 2 MX PCI

Quake 2 v3.20 (OpenGL / 640x480 / Demo01) - 34.5 fps
PC Player Direct3D Benchmark 2.10 (Direct3D / 640x480) - 35.9 fps
Forsaken (Direct3D / 640x480 / Ship Demo 400% speed) - 80.09 fps
Incoming (Direct3D / 1024x768 / Timedemo 1) - 28.51 fps
Expendable (Direct3D / 640x480 / go.exe timedemo ) - 17.4 fps
MDK2 Demo (OpenGL / 640x480 / timedemo ) - 20.93 fps

Intel 430TX / L2 cache 512kb / GeForce 2 MX PCI

Quake 2 v3.20 (OpenGL / 640x480 / Demo01) - 34.8 fps
PC Player Direct3D Benchmark 2.10 (Direct3D / 640x480) - 36.4 fps
Forsaken (Direct3D / 640x480 / Ship Demo 400% speed) - 81.31 fps
Incoming (Direct3D / 1024x768 / Timedemo 1) - 28.54 fps
Expendable (Direct3D / 640x480 / go.exe timedemo ) - 17.5 fps
MDK2 Demo (OpenGL / 640x480 / timedemo ) - 21.27 fps

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Direct3D & OpenGL tests (3dfx):

Intel 430HX / L2 cache 256kb / Voodoo 3 3000 PCI

Quake 2 v3.20 (MiniGL / 640x480 / Demo01) - 40.6 fps
PC Player Direct3D Benchmark 2.10 (Direct3D / 640x480) - 41.5 fps
Forsaken (Direct3D / 640x480 / Ship Demo 400% speed) - 79.87 fps
Incoming (Direct3D / 1024x768 / Timedemo 1) - 31.92 fps
Expendable (Direct3D / 640x480 / go.exe timedemo ) - 16.62 fps
MDK2 Demo (OpenGL / 640x480 / timedemo ) - 16.59 fps

Intel 430HX / L2 cache 512kb / Voodoo 3 3000 PCI

Quake 2 v3.20 (MiniGL / 640x480 / Demo01) - 41.6 fps
PC Player Direct3D Benchmark 2.10 (Direct3D / 640x480) - 43.0 fps
Forsaken (Direct3D / 640x480 / Ship Demo 400% speed) - 85.34 fps
Incoming (Direct3D / 1024x768 / Timedemo 1) - 32.69 fps
Expendable (Direct3D / 640x480 / go.exe timedemo ) - 17.2 fps
MDK2 Demo (OpenGL / 640x480 / timedemo ) - 17.07 fps

Intel 430TX / L2 cache 512kb / Voodoo 3 3000 PCI

Quake 2 v3.20 (MiniGL / 640x480 / Demo01) - 41.6 fps
PC Player Direct3D Benchmark 2.10 (Direct3D / 640x480) - 43.2 fps
Forsaken (Direct3D / 640x480 / Ship Demo 400% speed) - 86.94 fps
Incoming (Direct3D / 1024x768 / Timedemo 1) - 33.27 fps
Expendable (Direct3D / 640x480 / go.exe timedemo ) - 17.3 fps
MDK2 Demo (OpenGL / 640x480 / timedemo ) - 17.28 fps

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Conclusion:
Difference in L2 size is negligible for games. Apparently it's more pronounced with classic Pentium and low clock speeds, but with high clocked Pentium MMX it doesn't matter much. You can increase performance by about 5-7% with doubled L2 cache.
430TX chipset made some some improvements with PCI bus interaction, which helps some obscure 3D accelerators like Matrox Mystique. Other than that, there's very little difference between 430HX and 430TX chipsets even with different RAM, but 430TX is always just a hair faster.

To do list:
Add some ATX 430FX and 430VX results, if I find them or someone would be willing and able to do benchmark list above.
Benchmark Classic Pentium 200 for additional comparison.