Upgrade Help

First of all, you guys are great. I've seen how much of a help you've been to others so I thought I would come to this community with my problem. I feel the need to upgrade, however as I am no computer tech, I don't know what would be most effective for the buck. I have 100 ish to spend and the goal is to not have to worry about system requirments while gaming. Now I don't really care what the games look like, just to run them with a respectable fps.
Here is my system:
Stock: HPE-150t
Windows 7 Home Premium
Intel Core i5, 750 @ 2.67 GHz
6 GB RAM
ATI Radeon HD 5450 PCI

If you need more information let me know. I feel as though the RAM is the issue, but the ATI card is stock.
Not much of a budget, but I'd like to do the best I can.

Thanks Ya'll