If you're thinking of upgrading the CPU in the future, it might not be a bad idea to think about buying a new case (and maybe a motherboard) instead of upgrading your computer. You're not that far from building a whole new computer anyway. You can reuse the hard drive, optical drive and memory. I don't know if you can or should reuse the motherboard. Again I don't know much about replacing/upgrading the components on these computers so it may or may not be an easy task. One think I'm sure of is if you find that the GPU and PSU aren't compatible with your computer, then you have a reason to build a new computer. Here's a guide if you decide to go that route. There are more guides online and of course help from forums like Tom's.http://techreport.com/review/23624/how-to-build-a-pc-the-tech-report-guide
And here's another PSU for you to choose:http://www.newegg.com/Product/Product.aspx?Item=N82E16817207013
You probably know this already but I forgot to mention that you will be voiding your warranty if you're still covered. Lastly if you're going to upgrade the CPU, go with a quad core to see any real difference. And if you go with a K Series CPU for overclocking, you'd definitely need a new motherboard (Z77) to overclock and maybe an aftermarket cooler.