https://i.dell.com/sites/doccontent/shared-content/data-sheets/en/Documents/optiplex-990-spec-sheet.pdf
http://clascsg.uconn.edu/download/specs/O990.pdf
Best CPU you can use in that is the i7-2700 (non-K). It's not a bad chip, you'd get a slight speed boost & double the threads, & unless a newer game specifically asks for a newer generation you'd get decent performance in it.
Storage-wise, you've got the space to put a 2.5" 250+GB SSD inside there. Make that your OS, & turn your 500GB HDD into extra storage. If you have the money, you could also just get a 1-2TB HDD to go with the SSD.
RAM is a bit trickier. If you're using 32-bit Windows, you've already maxed it out. If you're running 64-bit Windows, you can go up to 16GB of RAM (only DDR3-1333 in speed, but as long as you can get to 8 or 16GB that won't matter as much).
The GPU is where it's going to suck the most. First, you're limited to half-height cards in the x16 slot (but at least you do have a full x16 slot, in addition to the x4). Second, that particular slot is limited to 35W. That's pretty much going to limit you to a GT 730 or 1030, as even a basic GTX 750 (sans PCIe connectors) needs twice the power. Unfortunately, they took all of the Intel HD iGPUs off of the hierarchy chart (https://www.tomshardware.com/reviews/best-gpus,4380.html), but even a GT 730 is going to be much better than your current Intel HD 2000. Neither the 730 nor 1030 will be great for games, but they'll do in a pinch for at least low/medium at 720p.