Hardy ever do i see it go above 20 FPS

So I am asking you all, what should I do to get better FPS...I'll even settle for 30 FPS if I can.
My system specs:
Emachines T6520
Windows XP Media center Editon SP2
AMD Athlon 64 Processor 3400+ 2.29 GHz
1.37 GB or RAM
ATI Radeon Express 200 Series (not sure sbout that one, its just what it says on the case

Thanks in advance!!