Take note that your mobo use's a 1.0 PCI-express x16 slot.And although 2.1 cards and 1.0 slots were made to be backwards compatible that their are some known issues with this.So my advice is that when your looking for a card make sure you get a 2.0 card.
No it's not performance it's just a general error between the 2.1 cards and the 1.0 mobo's.2.0 cards will work fine with 1.0 slots.It's just the 2.1 cards that are known to have issues with 1.0 slots.
Your PSU should be fine for a 6670/5670 or even a 5770/6770.
Is your budget pretty tight or can you stretch it to about $30 more? The 5670 is a nice card but I believe if you spend the extra $30 more you can get a 5770 and play those games maxed out or at least on high settings.
Do you happen to know what size your case is?Would you think it's a low-profile case or a Mini-Tower/Mid-Tower?
Actually all the games your looking to play aren't to particularlary demanding.You can easily get a 5670 and max out those games except SC2,I think you'll have to settle for medium settings for that game.
The 4650 is a very old card.I suggest going with a newer card from the 5xxx series.
I actually found a whole bunch of 5670's over at TigerDirect.