Is there any best gaming gpu's under 100$
Nothing that I would recommend on LGA775.
If you upgrade to LGA1155, you could get a Pentium G2120 for ~$90 but this would only be 20-40% faster than your current system so not really worth the trouble. The i3-3220 which costs around $120 would be 30-80% faster which is more worthwhile when you consider that you are already looking at a ~$225 rebuild cost ($90 CPU, $100 motherboard, $35 8GB RAM) even with the G2120.
Edit: scrap that, hallucinated 'CPU'. GPU-wise, the sub-$100 category is being eclipsed by AMD's A10 Trinity IGP, nothing worth recommending there IMO. If you want something that isn't obsolete vs IGPs, you have to look at the $150 price point.