7900gtx is a good choice, i picked up a 7950gt 512mb pci-e some years ago for a 98 build, really wasnt expecting much considering its an old card from 2006, yet im using it with win98 upto win11, it will also run some games released current year tho they may not be graphical masterpieces, still impressive none the less, latest ones i've tried were Yatagasaru Enter the Eastward and Under Night in Birth II Sys:Celes, i had to turn off a few settings for this one but at 720 it ran 60fps, not bad for a directx 9 gpu thats nearly 20years old.
if your installing on 98 theres at least two hacked drivers and if i remember correctly only one actually works properly, it should say tweaked drivers in white writing when starting the installer, the problem is since the driver doesnt officially support these cards it wont detect it and the setup will not continue.
the other way to install the driver is to extract the exe to a folder then go through control panel and tell it to search for the drivers there, i'd advise against this way as it just wont
work right and it can cause any number of problems from not having all available resolutions or being able to change to 32bit colour, or being able to change it then having it revert on reboot, the strangest one i've had is after installing and rebooting my desktop would flash up for a fraction of a second then my monitor would go out of range, after restarting a bunch of times and staring really hard at the screen i noticed the mouse cursor was rotated 90 degrees, well actually the whole desktop was? there have been times where it seemed to all work ok, though installing this way will only install the card and not the nvidia control panel which is essential.
the solution is simple though it didnt occur to me at first 🤣, if it wont install with a card it doesnt recognise intall it with a card it will, lucky i had an old 6200 pci-e which i use to run the driver once its finished it will reboot, when it posts before win 98 boots, i power off and switch for the 6200 for 7950 and it should detect the card and install it on booting win98, and you will also have the nvidia control panel, since this board wont do sound in dos i dont bother with Rloews NVSIZE patch, it works fine with 512 mb Vram.
Specs
Motherboard: GIGABYTE GA-G31M-ES2L rev2.0 (using LoneCrusaders Chipset Drivers for ICH7)
CPU: Core2Duo E5400 2.6GHZ OC@3.25GHZ (did upgrade to Q9450 but went back)
GPU: Nvidia Asus en7950gt 512mb ddr3 256bit PCI-E
Memory: 4GB DDR2 800mhz (x2 2GB) UC @ 667mhz for stability with overclock
Sound: Creative Soundblaster Live CT4760 (Liveware 3.0 CD)
Storage: 80GB 2.5" Hitatchi Travel star for win98 at the moment
PSU: Thermaltake TR2-550 PP
Media: Sony Optiarc AD-7241S Sata DVD burner (only drive i have that will burn cdrs as low as x8)
OS: Windows 98SE upto Win11 mainly Win7 SP1 x64 (Nvidia 309.08 for win 7/10/11 and KXaudio driver for win 7 & win10 version also for win11, i think it also has audigy support)
since this board wont do sound in dos so i have another that i use, originally it was a 478 socket pentium 4 2.8ghz Asus board with an 865 chipset paired with an nvidia fx5200, and onboard AC97, a solid choice for win98 for a first time build so dont discount it, it may not scream performance but in terms of driver support and out the box compatibility which is guaranteed, it cant be beat, also most intel pcs sold from 2002-2005 used this socket/cpu/gpu combo so parts will be plentiful and cheap, i did upgrade to an MSI 775 socket with a VIA chipset which i picked up on a whim for the bargain price of £2, i then spent £20+ to replace all the shitty 6.3v caps that were leaking, since its got official driver support for win95 upwards i thought it was worth it.
Specs
Motherboard: MSI PM8PM-V - MS-7222 ver2.0 (LGA775 + VT8237R Plus)
CPU: Celeron D360 3.46GHZ (Netburst, Cedar Mill-512, Single core with x64 Support)
GPU: MSI MS-8917 rev2.10 (Nvidia FX5200 128mb 64bit AGP)
Memory: 2GB DDR2 667mhz (x2 1GB)
Sound: Creative Soundblaster Live CT4620
Storage: 80GB IDE Maxtor DiamondMax Plus 9 & 80GB IDE Maxtor DiamondMax 20
PSU: Generic WinPower 450W
Media: LG Super Multi DVD rewriter GSA-H55N IDE
OS: Dualboot 98se & XP home SP3
this build works great for dos/98 games as well as games upto XP era on lower resolutions, ive also had it running 95 up to win10 x64. anyway as for playing at 1600x1200 at 60fps on ultra i think you'll struggle even with a 7900gtx which as far as im aware is the most powerful card thats win 98 compatible, i tried hitman contracts recently on both builds, graphic settings have no impact on fps only resolution does, on the 7950gt at 1280x1024 it was around 75fps if i remember right tho it would drop below 60 at times, on the fx5200 at 800x600 for the most part it was mostly always 30fps and it looked good and played great at that, some areas might see more fps, i tried Doom 3 on the 7950gt with the patch for 1280x720 and everything else on max, and it wasnt quite making 60fps i think it was 50-55. Prince of Persia Two Thrones from 2005 at 1280x720 made around 50-55fps, you may have some luck it depends on game and how well its been optimised, anyway good luck with your build lut us know how it goes!