As others have recommended the 2.3 is probably the best for just getting going. You still may want to pull the heatsinks and give in a good cleaning and put new silver on the CPU.

If you are handy, you can get either the 2.7 (Dual Core) or 2.5 (Quad Core). It will make a difference if you want to play videos with playback. My 2.7 is noticeably faster than my 2.3 with playing HD content. Get a fast card like a 1950 that also helps. You should pick up a donor 2.0 system to move the heatsinks over and delete the liquid cooling. I just did mine based on the videos and it worked perfectly.

The speed of these systems is really impressive. Good luck!