With 2GB theres very few current games that will run in the first place. You need AT LEAST 4GB to start, really 8gb is preferable. Then you should consider a good GPU. If you're willing to forego gaming at 1080p theres a lot of mid-low range cards for around $100 or so that will play most newer games on low quality.
The other thing to keep in mind is Win 7 32 bit is limited to 4 GB of memory, so you should consider updating it to 64 bit, or Win 8.1/Win 10 64bit , otherwise don't bother with 8gb.
At this point if you have to BUY Windows 8/10 then you should reconsider even building this PC. Whats your goals for gaming (as in what do you want to play)? You may be better served saving up for and building something newer.