This will sound really mean, but that computer is really weak. It's slower than my late 2005 desktop, which gets a stable 180-ish FPS at 1024 x 768 when I don't have any background processes trolling me and there's no SB or video. I cap it at 120 so my keyboard doesn't glitch, though. Now I use my new laptop which is actually a lot faster.
Save up for a new computer. You could get a dedicated graphics card, but that CPU's so old that you're better off just saving for a new computer with at least a third-gen i3 or better. The integrated graphics card in third-gen i3s and better will easily meet your FPS goal. However, what I would personally do is wait for the next wave of intel CPUs come out. If you start saving up now you should be able to afford a computer with one of those babies. You won't even need a dedicated graphics card to flat-out destroy your FPS goal though if you're getting a desktop you should eventually get one anyways.