A budget-friendly way that I found to get a new PC was to simply look for recommendations for parts that would work well together, get the parts at a pace I could afford, and assemble a new computer once all the parts had arrived. You might be surprised at how easy the assembly can be if you carefully read the manuals.
It's more of a World of Warcraft site, but MMO-Champion periodically posts recommended shopping lists for MMO gamers who would like to do exactly that. I did this last year and my new quad-core runs the game like a dream compared to the way my vintage 2007 single-thread PC struggled to.
Their latest recommended lists of parts that will complement one another well, with current pricing information and Amazon links for all the listed parts, can be found here
(you can scroll down past all the WoW patch notes and the PC builds are at the bottom, just before the comments section begins).