Best computer to use?

Please help me, I'm a hackin00b
I have a snow leopard install disc (I bought it officers, see? I have the reciept!) and an iMac worth about $650 on eBay.
I want to know, for $650, what gets me the following things, arranged from most to least important:
1) At least 2GB of Ram, preferably 4
2) Highest possible hard drive space
3) Compatibility with things important to media creation (USB ports, microphone jack, speakers.)
4) Highest possible processor speed
5) Best possible screen resolution (if desktop, ignore this)
6) Best possible graphics card
7) Highest possible HDD speed
Things I don't care about:
-Whether it's a desktop or laptop
-Battery life (it'll be plugged in most of the time)
-Size (as in portability, I like big screens :P)
-Disk drive compatibility
-Compatibility with anything not listed in number 3 above, really, other than things necessary for the computer to work
-Puppies