Get Xtech Systems X-Setup (6.1). It has virtually every tweak you can imagine for all Windows OSes including some XP-specific tweaks. It does everything that tweakUI does, but it has about 5 times as many tweaks. It will even (mostly) hide settings that aren't for your OS version, so you don't have to guess if a tweak will work or not. It's a pretty slick program over all, and the best part is that it's free. You can get it at www.xteq.com. Be sure to get 6.1, not 6.0 since 6.1 has the new XP tweaks.
 
My personal favorite is the ability to enable the undocumented manual crash feature. Just turn it on and when you hold Ctrl while you hit Scroll Lock twice, you get a lovely blue screen with the great message "THE USER ENTERED THE CRASH-DUMP" and then it reboots, lol Works in 2K, too!

Try http://www.winsupersite.com/showcase/windowsxp_tips_rc1.asp for RC1-specific tweaks.
 
Beta 2 tweaks are at http://www.winsupersite.com/showcase/windowsxp_tips.asp