16GB RAM?
I still use 4...

Get a bigger HDD if you must, a 256GB SSD since you should and proper graphics card since isn't yours kinda old?
I got a GTX 660 and it's really nice (from GigaByte, very quiet cooler on that thing) but a level or two above that can't hurt if you have the money.
Actually it might make sense to buy an SSD once you get a new Windows or reinstall anyway, saves you some hassle.