The power supply box should be up in the corner where all the cables are coming out from. (Or down in the corner, some are on the bottom.)
Thankfully DDR2 RAM is super cheap because it's not supported anymore.
You can buy it on eBay for like $4 a stick. You should have at least 4 GBs of memory for today's standard if you're running Windows 7.
Unfortunately without knowing your power supply I have to recommend you the safest card for your configuration.
But doesn't matter, anyways. Because your CPU is a little old and a newer card would create a huge bottleneck and be a waste of money.
I would highly recommend one of the GT Series cards such as the 440, 545, the 640, and the 740.
The GeForce GT 740 is the most powerful card that your system can scale to without causing a bottleneck or wasting your money.
HOWEVER~ Without knowing your power supply I can not safely recommend it to you simply due to it's power demand, as it requires a 400 watt power supply on at least a 20 amp 12v rail.
It's little sister the GT 640 is right behind it, and is only about 20% less powerful.
Thankfully they're an older, low profile card, and no one makes them anymore. So you can find them dirt cheap online.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814487147&cm_re=GT_640-_-14-487-147-_-Product
Here is a 1GB model for $59.99
They offer a 2GB variant, but they're harder to find, and are a tad more expensive.
http://www.ebay.com/itm/MSI-NVIDIA-GeForce-GT-640-N640GT-MD2GD3-2-GB-DDR3-SDRAM-PCIe-3-0-x16-/121493537562?pt=LH_DefaultDomain_0&hash=item1c4994431a
The 2GB 640 for $79.95
http://www.ebay.com/itm/Dell-0N3PJ-Nvidia-GeForce-GT-545-1GB-PCI-E-Graphics-Card-/301694388185?pt=LH_DefaultDomain_0&hash=item463e6307d9
Then we have the GT 545 OEM for $45. About the same power as the 640. They were put into Alienware Area 51 PC's some years ago, so they're hard to find.
All three of these cards will play games like CS:GO, BF3, and BF4, Skyrim, New Vegas, et cetera.
Just make sure you play around with the settings to get the best balance of performance and quality. (Like turning off anti-aliasing and lowering the resolution.)
My Advice: Get the GT 545 for $45 and buy some more ram. That motherboard has two DIMM and only supports a max of 4GBs of memory.
2 x 2 GB for 4 GBs total
http://www.ebay.com/itm/New-2GB-PC2-6400-DDR2-800-240pin-DDR2-DIMM-Dekstop-Memory-For-AMD-Motherboard-/231050735589?pt=LH_DefaultDomain_0&hash=item35cbb26fe5
2GB stick - $5.55 x 2 = $11.10
GT 545 - $45
GRAND TOTAL OF $56.10