Obviously the more ram the better. So just pack as much in as your budget can fit ha. Also, those Intel I3, I5, and I7 processors seem to be the latest thing. There is a dual core and quad core model of the I7. I you are going to drop the cash for an I7 or something similar you might as well get quad core because the price difference isn't that much. The performance boost is well worth it too.
Also, if you know enough about computers and have the time pick out your own parts and customize your computer. It's cheaper that way and you can hand pick the individual parts for your needs.
Here are some specs to shoot for:
-HDD 7200 rpm (preferable) Solid state if you can afford that. but who can?

-RAM at least 2 gigs. That is bare minimum. 4 or more should be what you aim for.
-CPU needs to be a dual core model (preferably quad considering how cheap they are now). Look for one with hyperthreading and 16 mb or more of cache.
-Also, front side bus for your motherboard is important. The higher mhz the better.
I hope I shed some light on a few things if you didn't know them already. I just went through the same thing you are dealing with now. I ended up getting a macbook pro only because portability is the utmost importance to me. If it weren't for that though I would spend about 400 bucks less and get a badass custom built pc that could run circles around this mac.
good luck!