your motherboard is going to limit you ALOT
you have One PCI Express x16 slot (PCI Express Bus SPEC V1.0 compliant).
which means any video card running PCIE 2.0 or 3.0 will run at 1.0 I would not spend a lot of money on video card with PCIE 3.0 in your case
suggestions will depend on the power supply you also have, could you identify the make and model of your PSU ?
the GT 730 is PCIE 2.0 so it would be a good card, covers the requirements set for CS:GO and fits your budget for new cards.
Here is a newegg.com link I like the EVGA one because its slim but if your not using an SFF case any of those will do the trick.
https://www.newegg.com/Product/ProductList.aspx?Submit=Property&N=100007709%2050001402%2050001312%2050001315%2050001561%2050001314%2050001669%2050001419%2050012150%2050001471%20600007610%204814%20600007782%20600519936&IsNodeId=1&LeftPriceRange=50%2070&bop=And&Order=PRICE&PageSize=36
you could go with a GT 710/720 for cheaper and upgrade your ram in the same time, would balance the system a little better for your gaming experience because 3GB isn't much to run on. considering your running a E5700 and the minim the game wants you to run it on is an E6600
ref:https://www.systemrequirementslab.com/cyri/requirements/pgl-2017-krakow-csgo-major-championship-mega-bundle/16284
CS:GO minimum requirements
CPU: Intel® Core™ 2 Duo E6600 or AMD Phenom™ X3 8750 processor or better
RAM: 2 GB
VIDEO CARD: Video card must be 256 MB or more and should be a DirectX 9-compatible with support for Pixel Shader 3.0