Resolume Avenue should run fine on any MacBookPro that has a dedicated GPU. That means that you'll get optimal performance with the 15" 2.66GHz configuration and upwards. Note that that is a recommendation, and that users have reported working fine with the 13" models as well.
As for upgrades, every user will rank different upgrades more or less important, but the consensus is that getting a SSD drive is definitely worth it. RAM is not so much of an issue, and you should be fine with the minimum configuration for that.
Also remember that you'll need to get a 3rd party scanconverter if you want to have analog video output.
Hope that helps, and good luck with your purchase.
Although technically VGA is indeed an analog video signal, a lot of people are using hardware that requires a Video or S-Video connector (like the Edirol V4 for instance). This connection is being phased out on modern computers, which is a gripe for most people buying the new series of MBPs.
If you're going VGA straight to the projector anyway, you have nothing to worry about and you should be fine!
