Which iMac should I get?

So I've run out of space and I think I should get more ram. I've decided I'll give my Macbook pro to my bro and get iMac, but I don't know which one should I get. I don't know if it's worth to get more expensive ones.
I'm using Logic 9 along with Reason and few plugins which are quite large, so I'll need some power to run it.
I tought instead of investing my money for Macbook I'll add it to a new one.
Thanks in advance.

I just got a 20" intel iMac that came out right before the new 21.whatever"'s one's and I run Reason, Digital Performer and Pro Tools w/ Waves plug-ins w/o any hiccups . Plus I just got Maschine and when I got most of my "groups" running it's only at 1/2 on the processor use gauge . Don't use Logic though so I can't tell you about that one .

How much ram do you have? Because my Logic started to lag after I bought legal copy of it, but there is much content in it so I'm not surprised in the end.

I was looking through it, and I don't know. Everything is leading to the most expensive one as it has 512 video card and I like to play sometimes. It is nice, but it's quiet possible new iMac will come pretty soon and it will be better obviously. I'm not much of a movie guy or something so I will mostly store samples and good music, so I 1TB should last long. Memory is expandable from 4 basic (which should be enough for now) to 16GB. On the other hand I'm in need of upgrade and this iMac will be capable to last for quiet long time and still stay strong. I heard thou this video card sux. The problem is I really like my Macbook pro and I'm not afraid of getting iMac. Mac pro is to expensive I think.
...looks like I've made my mind already, but then again I'd like to hear your opinion.
Cheers.

4 GB's ram