I am still mostly satisfied with my box that is running XP, 4GB RAM(3.5 USEABLE for you RAM nerds), some crappy AMD processor, and a $125 video card from 2 years ago. I can run WoW maxed out, Batman fairly robustly, most other Steam games flawlessly, and while SWTOR runs a little slow, I think that is more from my installing it on an external USB drive because I had no more space on my primary...
What does all this mean? I don't think you need to go all out on a PC to make sure you keep ahead of the slowly widening hardware requirements over the years. I have a pretty crappy system upgraded a few years ago, and I am satisfied. Of course, it may be more a case of me not being hip to the new gear and not knowing what I'm missing, but I still have my fun despite being a PC fuddy-duddy.