If this is a gaming PC then I would go X2 and then an X1950XT.
Read before you post:
I'll most likely be running Ubuntu (maybe dual booting Windows for some light-medium gaming). The system will be primarily used for general office, web stuff, with some video encoding (I still need to pick a capture card), and a little gaming - and I want it to be as upgradeable and long lasting as possible.
Besides, getting an X2 is not a matter of whether you're gaming or not. Athlon dual-core processors are good for budget builds, because they're very cheap and still competent. With a little more budget, Core 2 Duos are undisputed winners.
Also, I don't think I'd ever recommend an X1950XT for a gaming build (unless seriously restricted by budget). Such card costs $220-$250. The 8800GTS 320MB costs around $50 more, and it massacres the ATI card.