I think the requirements for Windows Vista/7 end up dramatically over-stated. They will run just fine on 2GB of RAM and a Core 2 Duo (I have two such machines, neither has a problem). Note that you will likely see increased memory usage under Vista and later, and that will also increase as available memory increases (up to a point) due to SuperFetch (and this is proper behavior). I probably wouldn't even bother with an SSD (as I've said before, I think they're a fad more than anything else (they aren't the "magic bullet" that most modern tech rags try to hawk them as)); you may consider ReadyBoost as it's fairly cheap, but as long as the hard-drive isn't a total dog it shouldn't be necessary (it can help with starting up big applications like Photoshop and game level loads, but just like an SSD will have no influence on anything that's computationally bound, or network/IO bound).
What IGP does the system have? I'd say if it's an Intel Gen4 or later it isn't worth bothering about (it will be AOK for everything but gaming (and it will actually handle some gaming too)). If it's a Gen3 (like GMA 950) you may consider upgrading that with a stand-alone card (there's plenty of decent options depending on what the system is capable of).
As far as the upgrades you've already committed to: the 6750 is a fine choice, and should be a nice improvement for multimedia-heavy tasks and such (I have a 6550 that's always been a champ, 6750 should be slightly faster), and the upgrade to 4GB of memory isn't a bad move either (especially if the system is sharing that memory for an IGP (some IGPs can dynamically allocate a reasonably large amount of memory, and Aero Glass will require more graphics memory than the XP desktop). If the system can run with 5GB (I'm assuming you bought 2x2GB and already have 2x512MB) that would be worth keeping assuming you're going with 64-bit OS (no reason to not do this), again especially if you have an IGP sharing some of the memory. I'd encourage, whatever you do, that you run the memory in dual-channel as the bandwidth gains will be helpful all around, and again, especially for the IGP.