I always prefer to max out my virtual memory in case anything goes wrong.
however, I have discovered that if windows doesn't have enough RAM, programs will crash or just quit all by themselves for no reason, dialogs won't display properly (buttons and other controls disappear).
If you can set it to 24GB minimum, do that. because of VM it will run slow, but with a sufficient amount it shouldn't crash. 2GB is minimum memory for 7. since XP required pretty much 128MB and started working optimally with antivirus at 1GB, optimum theoretically would be to buy at least 16GB. 7 is so new there isn't a lot of good metrics out there yet regarding memory, but I will look.
[I looked, and people run 7 with 512MB, but it was suggested that to test that you plug in all your peripherals. some have said that 4GB would be a good recommendaton for RAM. It is possible Microsoft has padded their System Requirements. in a microsoft forum, there was a hack given to make 7 run at 1/2 the memory, but that was RC or beta.]
this would be all new RAM, since it would probably be a different size than what you currently are using. I suspect your machine can take up to 24GB RAM if it is a new motherboard.
may I suggest corsair memory? if anything goes wrong, they will RMA/replace no questions asked. but you need to keep their sticker on it. put enough RAM in it and you can disable VM and make your system blaze. :-)
I choose to use a fixed amount because allocating and deallocating disk space for the VM not only takes up RAM, but it takes more time as well messing with the MFT. if your machine is running slow, it will run even slower on managed VM. If you use fixed VM, keep your memory usage down to a reasonable level - you will know when you have hit your limit when things start acting funny or crashing.
at 3GB, you probably don't have much to work with.