What locked fps would you suggest for my system?

1.8ghz, Intel 2 dual core, XP sp2, 1GB ram, 512mb ATI radeon graphics.I had it looked at 15ps before SP1.The graphics come up grainy when I put the settings too far to the right, and I have found 53% traffic (mostly pai) is good enough for my needs. I have sparse autogen, as again that takes up a lot of memory.Please what suggested locked fps should I go for? I can't have unlimited as my system can't cope (no CTD though) and it seems blurry and juddery.