No you do not need a DDR2 card. I believe my graphics card uses DDR5 and my motherboard uses DDR3.
You basically just need to make sure you have the right style of slot.
Is this your motherboard?
If so you just need a PCIe card. This is the most common type.]
How much are you willing to spend? That's all it really comes down to with graphics card. Want more power? Spend more... I chose the HD6870 because it has a great price/performance ratio. You can find it for just over $200 now. It can play essentially any game out there right now at high settings.
The HD5770 is a good choice if you want to save some $ but still get a decent card.
EDIT: and as the guy above me said, you need to consider how powerful your power supply is. But considering you only have a dual core CPU I don't think you will need a monster card that will consume a ton of power. If you have a halfway decent power supply you should be fine with an average "gaming" graphics card like a HD5770.
If you were to get a expensive high power graphics card like a GTX 580 your CPU would just hold it back...
Other choices I suggest checking out include the GTX 450, GTX 460, GTX 550, GTX 560, HD 6850 etc.
Those are all mid range cards with decent performance that will run games at a healthy resolution and frame rate.Edited by Epitope - 5/25/11 at 10:52pm