you have a very good video card so it can't be that, the CPU is definately fast enuff especially considering the load taken off it by the 9800
sooo much depends on the RAM, and the cheapo brand may not be performing well, bottlenecking it
I suggest getting 2x256/512MB PC3200 (400MHZ) Corsair Micro or Kingston
I'd go Corsair personally because it has such low latency, and comes with heat spreaders