Which Graphics card should I buy?

My current graphics card is an internal NVIDIA GeForce 6150SE nForce 430 with 128MB video memory.
With 128MB video memory, Ubuntu unity lags as well as MTA and TF2. 720p videos work fine but sometimes they lag too. Overall I experience frequent FPS drops with my current internal graphics card.

So I am planning to buy a new one, my budget ain't high, I am thinking of getting a NVIDIA GeForce GT 630 Synergy Edition 2 GB DDR3 available here ZOTAC NVIDIA GeForce GT 630 Synergy Edition 2 GB DDR3 Graphics Card - ZOTAC: Flipkart.com for 3900 INR, i.e 67.69$. Also I learned that a DDR3 graphics card would work with my motherboard which only supports DDR2 memory, is that true?

Another one I considered is : NVIDIA 2GB DDR3 GT610 2 GB DDR3 Graphics Card availabe here Gainward NVIDIA 2GB DDR3 GT610 2 GB DDR3 Graphics Card - Gainward: Flipkart.com for 2563 INR i.e 44.49$

Both of them have 2GB memory which is far better than my current 128MB video memory. I cannot decide on which one should I purchase. Also consider recommending other graphics card than these two.
Overall my budget is below 70$.
My system specs are available below.

I think by you should buy GT630 or GT640 for your system specs, you should go for GT640 cause your CPU provides
1 * PCI2 * PCI Express x1

1 * PCI Express x16 slots, so u can go for GT640 or I think u should by GDDR3 / GDDR5 cards for high end gaming...

All the GT640 series cards are above 70$

Ednan, yes a graphics card with DDR3 will work with your system. Vram is different than system ram. The GT640 is more, but would increase performance.