On the amount of RAM on #3, I'm assuming it's running VISTA. Of course, the others could also be running VISTA as well.
I would normally select #2 even though I'm an AMD person, but since it has an integrated graphics chipset, I'll have to go with number 3. BUT, the graphics card has to go. I'm also assuming the graphics card is a PCI-e based upon it being an AM2 processor and board. Since you didn't state what brands these are, nor did you state if you are going to use them for gaming, I'm just going to go with the flow. If you are going to game with it, AND you select #2 anyway, (and assuming again, that it has a PCI-e slot), you'll still have to install a new graphics card.
But, if you go with #3, and the additional $300 isn't an issue, for what you'd save on purchasing #3, you can pick yourself up a really decent Direct X10 card. The 8800 GT's seem to be a popular, high quality card right now, so that's just my opinion. If you're not using it for gaming, then definitely go with #2.