I don't know what XT stands for, extream I guess but the cards with that lable are hardly extream. XT cards are much slower than their regular and Ultra counterparts. You would have been better off buying a FX5700 or a regular FX5900.
The FX 5900XT is a crippled version of the FX 5900. It generally uses slower RAM but the nv35 GPU chip is the same. The PCB design is usally modified from the other reference FX 5900 cards. Depending on the manufacturer or luck, it may even turn out to be a performance card with quality components onboard.
The stock clocks on the 5900XT are: GPU=390mhz / RAM=700mhz. Mine clocks fine at 450/900 and runs reasonably cool at that. Going to 460/900, I see the temp rising. Winfast provide the overclock utility for my card.
Even at stock clocking, the FX 5900XT isn't very far behind it's standard FX 5900 counterparts.
Any FX 5900XT card blows away all FX 5700 cards, that's just a fact, the FX5700 GPU just isn't in the same league.
And yes, Gixer, it does handles the AA, AF and higher resolutions better.
I noticed your system overclock Gixer, I too just upgraded to a XP2800+ and obtained 364mhz FSB with no apparent heat rise, scored just over 16,000 3dMarks in '01 with the card overclocked as above. I had to add a case fan to blow across the video card. There are no heat sinks on the Video cards RAM chips, but the RAM is running cool enough, it's the GPU that is warming up.
Look at Leworthy8's score with his overclocked XP3200+ !!! ;) Well done!

Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&