Don't bother with that stuff if it can handle most of what you do now. Save your money for a few months and by the time you have enough the low- to mid-range Intel Nehalem parts should be on the market. Even the low-end Nehalem chips will destroy your current Athlon X2 and give you far greater performance in Tremulous than another HD 3650 would.
After you get the new motherboard, CPU and RAM (Nehalem is DDR3 only) invest a new hard drive. I recommend Seagate Barracudas, Samsung Spinpoints and Western Digital Raptors. And finally, prices on the HD 4870 should be well below the $200 mark - get it while you can.
Obviously that could all change, computer technology evolves so quickly that it becomes difficult to decide on something because you know it'll be outdated within the year. But if you don't want to wait and want a few small upgrades now, do what Bissig said: New HDD and an ATI HD 4850 (the latter can be had for as low as $139 right now).
