Your processor, which is realistically the worst part of your pc to upgrade. Since whatever motherboard platform you are currently running is probably defunct, it would be an awful investment to purchase a new processor to drop in. I can give you a modest build off of newegg that would cost $450 or so that would be top of the line, but your best bet is modify settings you're currently using.
If you can stand not playing in fullscreen, toggling on hardware acceleration makes everything amazing. Furthermore, you can change quality to low, disable projectiles of allies etc. There's loads of tweaks. Also remember to refresh the game each hour.