upgrade the ram and gpu and you're set.
i used to play source and 1.6 relentlessly on a p4, 512mb, radeon 9200 (agp8x?i think)
I could get a solid 101 fps in 1.6 with the proper tweaks and about 40-60 in Source..after time source actually become a bit smoother..due to Valve/steam caring about it.
