Thats not hard to anwer, maybe a better gfx card. If you have the money, get a new computer. Celerons won't do much for gaming, 1GB DDR PC3200 would also help.
what kind of upgrade will i have to make if i want to play games like hl2 or doom3 in medium quality but good frames per second? my pc can play farcry but barely. i have a hp pavilion a809rs. its has celeron 340 2.93GHz 256KB L2 cache 533MHz front side bus, 512 MB ddr sdram PC2700, and intel 845gv with 64 MB allocated video memory.