According to this:-http://www.newegg.com/Product/Product.aspx?Item=N82E16883114067
you do have a PCIex16 slot - but the best way to be absolutely sure is just take the cover off & have look.
The GT730 you are looking for has a low power requirement & has no power connectors (other than the PCIe slot) the recommended power supply (http://www.geforce.co.uk/hardware/desktop-gpus/geforce-gt-730/specifications
) is only 300 Watts. You can probably even get that to run on the stock 250W power supply.
I don't think you're going to be able to upgrade the CPU enough (on the existing motherboard) to meet the CS GO minimum system requirements so it's probably better to save your money & put it towards your "full on" build. Otherwise you will need a new motherboard (micro ATX) with CPU & so probably new RAM and also the new PSU already mentioned - but a plus point is that a new motherboard with current integrated graphics might be OK for CS GO without the need for a separate graphics card.
Edited by rqt, 11 February 2016 - 04:24 AM.