And it's a crappy 64 bit GDDR3 900MHz memory clock card?!

GeForce 4 Ti 4600 had around ~10Gb/s with just 128-bit 325 Mhz DDR1 memory.

That's how it works, a GPU's RAM is much faster than the bus it's on. However most data, such as textures and meshes, will be uploaded to the GPU at a strategic time (loading a map) with only specific dynamic stuff being sent over the bus during gameplay. Especially effects that depend on reading back from the GPU can cripple framerates.

So if the 4600ti has a max bandwidth of 10GB/s and AGP x8 can only do 2GB, would a PCI-E version of the 4600ti get significantly higher frame rates?

Bandwidth affects loading times not frame rates in general. Once textures are loaded that's it.

Almoststew1990 wrote on 2020-02-29, 10:30:

So if the 4600ti has a max bandwidth of 10GB/s and AGP x8 can only do 2GB, would a PCI-E version of the 4600ti get significantly higher frame rates?

Just to point out the obvious : GeForce 4600 Ti does NOT support AGP 8x - it's AGP 4x only card.
What GPU gets from CPU, is only the raw data and instuctions what to do with it.
The work itself (based on provided data) is done 100% internally, unless GPU runs out of VRAM (or something will require additional data from RAM).
Either way : PCIe speed won't be the main issue with your card.

Think of it this way :
In CPU-GPU, time needed to tell GPU what to do (including time needed to get the stuff it needs), is usually a lot less than ammount of time that it will need to complete given task (in short : if CPU is fast enough in giving direction, GPU isn't fast enough to keep up with it).
^this is basic summary of how CPU and GPU operate/interact.

See this very informative article:

https://www.techpowerup.com/review/nvidia-gtx … xpress-scaling/

In a nutshell, on average cards:
PCIe 2.0/3.0/4.0 16x delivers full performance (2.0 may drop 1% on newest generation)
x8 will see a drop of 2% to 6%. You will most likely not notice that.
x4, most probably only 2.0 on your MoBo, will drop between 6% to 14%. You will notice that on games with already low fps

Framerates will drop on x4.
Because rendering (among other things) takes place on the GPU, higher detail-setting will not affect this.

You'll be fine. Ages ago I did some comparison of PCI-e 1.0 speeds. 16x PCI-e 1.0 was faster than 4x PCI-e 1.0 but not /that/ faster, using an HD6950. PCI-e 2.0 4x is the same as PCI-e 1.0 8x so I doubt you'll even notice.

On the other hand, PCI-e 1.0 1x is slow as dirt (much slower than AGP 2x), gaming on it is painful.