There's ton of things yeh can tweak.

Here's what I run a p4 with one gig of ram some eons old pci graphics card.
I can run 3 with minimal lag at low setting and can do up to 4 in an instance with a bunch of lag.

start-control panel-Administrative Tools-system click advanced tab-performance settings and click adjust for best performance.

Remove all those thing that auto-load in "start-all programs-startup"

get rid of all the extra stuff yeh dont need.
start-control panel-Administrative Tools-services sort by started
yeh can google each one to see what it does and get a good idea what to remove.

does items will net yeh the biggest returns.

do you have maxfps and maxfpsbk set? I use maxfps 20 and maxfpsbk 5