Check out the Tom's hardware cpu hierarchy chart:http://www.tomshardware.com/reviews/best-gaming-cpu,2466-6.html
The author doesn't recommend an upgrade of less than 3 tiers, but the 4200+ runs at 2.2 ghz and the 5600+ runs at 2.9 ghz. You probably won't see a huge difference. If I were you, I'd save my money and think about a whole new system. There are some real bargains out there right now, especially with AMD processors, that are far superior to the 5600+. Good luck.