I have 2GHz Mini and it is plenty fast and I need it to do more than what you do. So you'd be fine with getting the new one or the old one.
Really... it's up to you.
For the record, I don't really care for the 2gigs of RAM I have and will be soon getting 4gigs.
Edit: For those saying the power brick is a pain... Here's my say on that. What I have, essentially, is a small entertainment center in my room. So I have surround sound, 2 monitors (1 being a 32 inch Sony Bravia), a Xbox, the Mac Mini, and soon a Blu-Ray player. That said, if you're running your Mac Mini next to an Xbox, or anything else with a brick for that matter, then you really won't notice it being there because you already had one in the first place. That's 100% my opinion though.
Would it be nice for me not to have a brick? Yes. Would it make an impact on me if I didn't have it? No
The only difference the non-brick 2010 Mini would have on me (as far as the power supply goes) is aesthetics. And my brick is hidden behind a bookshelf anyway!
I'm not saying it's not going to be a problem for everyone, I can see where it would be with limited space. I'm just saying, if you have the room for it don't make that one of the reasons not to get a refurb.
Power Consumption: Here is a change that may end up costing you. The 2009 minis used 110W (max) whereas the 2010 minis use only 85W (max). That is nearly a 25% decrease in maximum power usage! (If your computer isn’t running at full speed all the time, and it probably isn’t, it might not be as extreme, but there will still be a savings.)
As mentioned above that may be something you may want to consider. I don't pay the bills so that doesn't affect me at all. (Dad does)
Also, I turn my computer off every night before I go to sleep, and put it in sleep mode when I'm not using it. Personally, I think it's better for the computer because I'm also anal-retentive but it also saves energy while doing so...