How much do you think you'll get out of your SR? Trouble is, a lot of people know about the issues of that system, and it may effect what you'll get out of it unless you find an unsophisticated buyer on CL.
A local used computer dealer in my city was selling early 08 base model 15" mbps for $800, with a one year in store warranty. I'd check out what your model is selling for in your area on CL.
There's a lot of factors to consider. Why do you want a new system? Does your current one have applecare?
If you're worried about a video card failure, and want to unload it before your applecare/extended apple warranty of the video card expires, sell it if you can get enough money, or boot camp windows and game the s#^t out of it (Apple will have to run out of replacement logic boards at some time!).
If you're looking for a little more performance, replacing the stock HD with a larger and/or faster one is a valid option. Assuming the video card holds up, you probably can goose a little extra life/usability out of your existing machine.
If you're wanting to keep up with the latest features/technology, you're playing a losing game.
If it's extra portability, then the 13" is the way to go.
If your current system is doing what you need it to do (and are willing to gamble on the graphics card), you should keep it going as long as possible and save your money in the meanwhile for a new machine.
It's not an easy choice either way. Good luck!