Just rooted my phone, can I get 2.1 and stay rooted?

I looked here I couldn't find a direct answer to my question, so I'll ask it here:

I just nervously rooted my phone using http://www.youtube.com/watch?v=WAi--u4y61Y"]this guide so now I have 2.0.1 software, but it's rooted! I was wondering if there is a way to get back to the official 2.1 software and stay rooted. I saw a sticky thread at the top of this forum (after I rooted) for rooting from 2.1, but since I'm already rooted, I prefer not to do it again. Also, I'd like to eventually install a cool looking ROM like Darkedge, so are those strictly themes (looks) or do they effectively update my phone to 2.1 when I set them up?

The main reason I rooted my phone though is that I would like to overclock it. Does overclocking stay even if I update or change ROMs? or do I have to do it again each time? Thanks!!
http://www.youtube.com/watch?v=WAi--u4y61Y"]