You can get the student price, so a 15" with discrete graphics starts at $1850. You could also try the refurb store or get a last-generation model somewhere.
A 7200 RPM drive will be helpful, and you will probably save money if you upgrade it yourself. You will also want a large drive since video files take up a lot of space. You will probably want 4gb of RAM.
I don't know how much discrete graphics will really help you. As far as I know, the video card is only used to render realtime previews of some effects. I tried to get GPU accelerated encoding working under Windows, but the results weren't very good (the ATI software didn't have many options, and the CPU still still did most of the work). I don't think GPU accelerated encoding is ready for primetime yet. OS X has Open CL, but I don't think there is anything out there that can take advantage of it yet. If you decide you don't need discrete graphics, and since you have an ACD, either of the 13" models might work for you. The 2.53 should be 12% faster for most tasks and up to 24% faster for completely multithreaded tasks. I don't know if Final Cut is multithreaded, but I would guess it is.
There is essentially no way to fit a discrete card in the 13". There isn't physically enough space on the board, the cooling system wouldn't be able to handle it, and the battery life would be terrible. If Apple took out the CD drive, they could probably do it, but I don't think they are likely to try to fit discrete graphics in a 13" any time soon.