I use an i7 920 with 12g of ddr3 and a gtx 270. I 5 box with pretty good fps. My tips are thus:

1. In ISBoxer, turn on "Make game think it's the foreground window always" option. This boosted my FPS a bit.
2. In the video settings turn EVERYTHING down as far as it can go, and THEN turn on the low quality renderer. Log out with everyone and then log back in. This alone caused me to gain 10fps.
3. I set the main window to cores 1-4 and then each other window gets one core each.