I used to be in your shoes...trying to get into pro 3d (difficult), with a mac (more difficult), and all on a students budget (OMFG type of hard).
I will give you some hard learned lessons.
1) Memory and Vid card are more important than processor.
The processor only controls the render speed. A used computer with a C2D (Core 2 Duo) will be slow, but it's OK to learn on.
Unless you have AT LEAST 4gb of RAM, the screen will bog down/freeze when you are working with too many characters.
8GB seems to be a good zone. It's also the max that any mac in your price range can accept (unless you have an unlimited budget). 8gb of memory goes for about $60 these days. But hurry, last year they were $120.
Unfortunately even a used mac mini goes for $400 these days.
Last winter I sold my 2 yr old C2D mini for $400 on criaigslist.
Judging by the 12 e-mails I got within 30 minutes, I guess that even $400 was on the cheap side.
2) The new (base) mac mini isn't good enough. I would have said that the $599 base mac mini would be OK, since I have used earlier C2D mac minis' and C2D MBPs' with C4D (and 8gb of RAM...hint hint). But now I see that the new integrated graphics is actually much SLOWER than the old integrated board. So I cannot, hand on heart, recommend the new (base) mini for anything other than learning on. Especially not if getting to that price point is going to be a sacrifice. If you can get together (read: beg parents) $799 for the upgraded mini with the Radeon graphics chip (plus $60 for more RAM + sales tax), then you will be good.
3) Otherwise, it'll have to be a hackintosh. For $500 you should be able to build yourself a decent little system.
But note that it will basically be the same as the $799 mini...but bigger and louder. MUCH louder. Macs are basically silent, towers are NOT.
But you can overclock it.
You might get a 3.5 on the cinebench CPU test.
(A standard 12 core MacPro tower will get a score of 15-16 straight out the box.)
Hackintosh.com has parts lists with prices and links. They also have vids with step by step instructions on how to put it all together.
It's actually not that bad, it's kinda fun planning your perfect system to maximize your budget.
I would suggest that you take a few weeks looking at youtube vids of custom computer builds and hackintosh tuts. Go to tigerdirect and look at all the cheap prices for computer parts and start dreaming/scheaming.
But warning...you might be lured into thinking that " maybe Windows isn't so bad after all" maybe they "finally got it right with Windows 7".
Don't believe their lies.
4) The biggest problem is figuring out your program pipeline.
Cinema 4D is the best 3D program which has always been loyal to mac users. Any update that they make goes to macs first.
Maya 2011 has just started porting their program for macs a few years ago, but a lot of the third party plugins still don't work for mac users.
But Maya is the Gorilla in the room, it will always be an asset to learn it. Plus Mudbox is seamlessly integrated with it.
Modo is a great modeller, but so is Silo, and Silo is cheap.
If your really poor
I'd say buy Cheetah3D and start from there. The interface is a LOT like a baby C4D, so what you learn with Cheetah can be used when you move up to the big boys.
