Turn off unused system services.
No, really. Even though they are not "using CPU time", they still occupy a chunk of vital resources - RAM. Disabling the service typically results in the program being unloaded from memory, and that little bit of memory being freed up. Go to your control panel and open up the Services icon in there - see all those entries? Each one that says "Enabled" means something somewhere is gobbling up some precious RAM to accomplish what you're after.
This may sound like a futile exercise, but I have used it successfully to play games that "required 1Gb of RAM", on a 512Mb system. The real requirement wasn't 1Gb, it was the fact that you needed that to comfortably run the game. Turning off system services freed up a whopping 120Mb of RAM. The game loaded and ran just fine.
You'll want to leave those services that you absolutely need - DHCP Client, DNS Client, etc., as they will be required to keep your internet connection stable. But things like file and print sharing, well, what are you going to do while you're blowing crap up in the game? Print a spreadsheet? Turn off Workstation and Server services to start, and go from there.
Look at flashing to the latest BIOS.
Video drivers do have an impact, and updating the Video BIOS will help, but your Motherboard BIOS has just as big an impact. Many users don't even bother to upgrade to the latest versions, which sometimes include enhancements to how your I/O works, options that you can enable that weren't there before, and bugfixes for those options that didn't work well. Consider the pros and cons of this - while it will allow you to run a newer BIOS, there is always a little tiny risk of a bad flash. If BIOS updates are available, read the update READMEs and decide if there is anything in there that would improve your system performance.'
Compromise on graphics performance
I know you said you don't want to turn down your graphics options, but have you considered optimizing them? Sometimes, leaving all of your current graphics options enabled but switching from 32bpp to just 16bpp is all it takes to increase your performance. You get to keep your current screen resolution and have all of the eye candy you want, just with 65535 colors instead of 16 million+. Sometimes it's not the big tweaks that gets you the gain, it's the little ones.