I recommend that you use as little as possible. The big mistake that a ton of people make is loading their device with so much crap it slows to a crawl. I have seen posts where people spend an insane amount of time trying to get a weather app to run properly. Weather? Really?

I play MP3s, watch movies, I get emails from work with attachments, edit them and send them back with revisions and I play a lot of games. I do it all with the default WM6 (6.1).

The big mistake people make when loading down their system is tweaking their interface. A neat new background? Cool. Stay away from anything that takes over your entire system.

And the answer to your question is stickied and answered to death.