I own a Radeon X1900XT 512mb. It's an excellent card, and it enables me to run all my games so far at 1600*1200 with at least 2xQuality Adaptive AA(I usually leave adaptive off for more performance as I usually see no difference in quality at all, besides, you aren't going to be paying attention to the aliasing in the fence during the heat of battle anyway), and 16xHQAF. I hope to ride it out on my X1900XT until 2009, when I can get a DX10 card.
The X1900XT should last me until then, providing DX10 doesn't become mainstream too fast.
But, for now, my X1900XT is excellent.
I also have a BFG 7800GT OC 256mb in my closet, which was a great card(It was quite a step from my EVGA Geforce 6800 128mb AGP that I came from, but then again, I also was upgrading from an Athlon XP 2600+ and 512mb of ram to an Athlon 64 3500+ and 2 gigs of ram, so I can't say for sure how much the 7800GT improved gaming on its own, but I still got great performance out of it) but I saw the X1900XT back in January and decided it was worth an upgrade. It was the High Quality AF that mainly attracted me to the X1900XT.