Troubleshooting hardware related problem is a very frustating endeavor (user level) for the simple reason that you do not have any spare part at your disposal. You must rely on guesswork (unreliable). Eventually you'll replace the defective/incompatible componet(s). Money and time will be involved. Probably too much of both!
In your case, don't assume the video card is defective and please don't spend any money on hardware before you can isolate the problem. This basically mean, NO UPGRADE of any kind, period! No faster ram, no beefier power supply. You might be surprised at what is actually causing your problem.
I will assume you were thourough in installing the video card; properly seated in the AGP slot, auxilliary power connected to the card etc. I will assume the same for the computer itself; CPU speed and BIOS settings are at default value, well built, installed and maintained. Since you did not experienced any problem with your previous card i will ASSUME (always a mistake but...) that the computer is OK. Go over the ASUS/Saphhire manual/website for any related small print info regarding any incompatibility that could be resolved with a BIOS upgrade or other. If nothing is mentionned then you might want to try the following steps:
Test your card in a friends' computer.
This might involve a little bit of hassle for your friend; uninstallin/installing drivers on his/her system and the possibility of screwing up his/her Windows installation. Take the neccessary precautions; Ghost image file or XP System restore etc. You might want to spring for beer and pizza afterwards. Note: your friends' computer must not be underpowered.
If you bought your card at a local shop you might want to ask them to test it. Tell them that it is not performing adequately in 3D games. Tell them what steps you took to try to fix it; fresh install of Windows, different drivers, asked questions on Help Forums etc. Ask to be present when they do it; used to do it all the time when i did this kind of work (clients appreciated). Bring your copy of UT2004 and ATI drivers with you. Nothing more annoying then a customer who brings in a troublesome component but not the software/drivers along with it. If you can bring a printout of the website UT2004 benchmark that's always a plus.
DO NOT have the video card tested with the same brand/model/chipset of board you are using; in this case ASUS (this applies to the friend or the shop). If it performs badly also at the shop, using a different brand/chipset motherboard audio card combo, then it might need more investigative work, an undiscovered driver bug or just a plain old defective card . Good luck.
Countach69