Just so you know, ATi has a trade in program that will allow you to trade in old cards for pretty good refunds or credits.
Also, I really think that with a bit of tweaking you can get a lot more than 8fps out of Doom 3, I just don't think you are trying hard enough, or something else in your system is bottlenecking it. While I do agree that a 6800GT is the better card, you can get much better results out of x800 with a bit of tweaking, there's no bloody way I can more FPS with a 9800Pro than an x800.