I have both a Ti4600 and a Ti4600-8X (aka Ti4800) that I've tested in my ABIT KT7A v1.0 motherboard with an Athlon 1400 Thunderbird and 512mb PC133 RAM running Win98SE.
(Please keep in mind that the KT7A only has AGP 4x)
They both perform identically and can get me ~9000 in 3DMark2001SE on a good day.
I also have an FX 5950 Ultra that I run in my ABIT KT7A v1.3 motherboard with an Athlon XP 2100+ Palomino and 768mb PC133 RAM running Win98SE. Interestingly, it only gets me ~8900 in 3DMark2001SE. I do not know why and I haven't swapped in my Ti4600 cards for comparison. Why? Well...
... as others have mentioned, the Ti4600 and FX 59xx cards are serious overkill for any Win98SE/DX8 system. There isn't a single game that suffers under either of those configurations. So, I just haven't bothered troubleshooting to find out why my supposedly superior system benchmarks lower. Maybe one of these days I'll figure it out.
(As somebody wiser than myself once said: "You don't build the sytem for benchmarking, you build it to play the game.")
Until then, here's my opinion on the Ti4600 vs the FX 59xx cards:
You can find both of these on eBay for under USD$25 if you're very patient. If you're in a hurry, you will pay more for convenience.
The Ti4600 and the FX 59xx both offer very similar levels of performance. The difference is that the FX 59xx card gives you better FSAA options.
(The FX 59xx cards will probably benchmark higher with CPUs and RAM faster than what I run.)
(While I haven't compared an FX 5900 Ultra to my FX 5950 Ultra, I expect the performance difference would be negligible.)
If you're strictly after a high FPS count, then stick with the Ti4600 and the greater flexibility it offers with legacy drivers. If you want better FSAA, then get the FX 59xx card and accept that your driver options are slightly more limited.
If you want the FX 59xx card and run anything less than a P4/Athlon64 ~3ghz, get the 5900 Ultra. It runs a bit cooler and you won't notice a difference in performance anyway. You can probably find it cheaper too.
Note:
Ti4600-8X (aka ti4800) and FX 5900/Ultra requires drivers v45.23 or newer.
FX 5950 Ultra requires drivers v56.64 or newer.
(The earliest version I run nowadays is v45.23, so I don't mind very much.)
(I find v45.23 to be the ideal Win98SE driver.)
Regarding the limited availability of Ti4600-8X/Ti4800 cards:
Don't search eBay for Ti4800 cards. You'll likely only find the silly Ti4800SE, which I myself was scammed by at one time many years ago.
(The Ti4800SE is merely a Ti4400 that can run at 8X AGP. But, why bother?)
Expand your search to include all Ti4600 cards and look at the pictures carefully. Look for "Ti4600-8X" printed on the stickers on the card. That indicates that it's actually a Ti4800. Even though the card's sticker identifies it as being a Ti4600-8X, the NVIDIA drivers will detect it as a Ti4800.
I obtained my Ti4600-8X/Ti4800 by accident. I thought I had purchased a regular ol' Ti4600 with a broken HSF. At the time, I was running v30.82 drivers and it wasn't detected by the system correctly. Upon further troubleshooting and upgrading to v45.23, I found it was detected as a Ti4800. More research revealed that the Ti4800 was usually marketed and sold under the standard Ti4600 moniker and sometimes as "Ti4600-8X". A sticker on the back of my card showed "Ti4600-8X", thus confirming my card as a Ti4800.
Edit:
While I own a copy of Splinter Cell, I've never played it. I have no idea if it would work with the FX cards or not.
Edit edit:
The Ti4600 also has an advantage in that it only occupies a single slot, unless you get crazy with aftermarket coolers. The Evercool VC-RF and its various rebranded clones offer great performance and will only use up a single slot if you don't bother with the ridiculous cover for the HSF.