If your budget is truly stuck at 100, that sucky gt520 is holding you back WAY more than your cpu would...get a HD7770 or HD7750 (slightly weaker but doesn't need power supply plug) or GTX 750 superclocked (Also doesn't need power supply plug)
then when you can, a core 2 quad is cheap on ebay, or at the very least a higher clocked core 2 duo, but don't expect too much. Save up for an Athlon x4 750k (often recommended by tom's hardware in their best gaming cpu's for the money) and cheapish but decent fm2 or fm2+ motherboard at the least....Ideally on a budget though you'd want haswell i3 or AMD FX6300 6 core
there's no getting around it, your setup is very obsolete, but, getting a 100 dollar graphics card can extend it's life, and you can use the new GPU in your new build later on.
You can often find the HD7770 GHZ edition under $100 too, yesterday newegg had the r7 265 or r7 260 for like 80 bucks after rebate
or hit up the deals section here, some guy is selling a gtx 560 for 60 bucks and a fx4100 and mobo combo for only 60 bucks, even that would chump your current setup (no offence just facts)