You can check the Avid certified configurations at https://www.avid.com/mc.
As far as I can tell, you have a pretty decent machine, and it’s on the list, except for the graphics card. That does not mean it’s going to be a problem, especially since your card is 512MB, chances are it will run just fine. The one officially supported for your machine would be the GeForce 7300 GT (which is 256MB). If it turns out your graphics card is giving you trouble, then you could put that one in (I don’t think it sells for more than 70 bucks now).
If there is indeed going to be a new release within weeks, and you buy MC 3.5 today, it would be very, very likely that you will fall in the usual “grace period”, meaning that this new version would be free of cost for you.
The thing that you should worry about as well is the right OS/Quicktime combination. For 3.5.4, Avid wants you to be on 10.5.5., 10.5.6 or 10.5.7. 10.5.8 has not (yet?) been qualified. Might work, might not. QT should be 7.5.5 or 7.6 (not 7.6.2 for instance). Again, using different versions than specified might work, might not work. If you’re running different apps, you might want to create a separate drive or partition running the right combo of Avid, OSX and Quicktime.
But here’s the even better news for both your questions: you can download a 14-day free, fully functional trial version on the Avid website. This will allow you to install and try the software on your machine, and test if it works out. I’m betting it will, but why not take the free test ride?