Again, not much in gaming. However, if you switch to an Intel CPU, you will be able to SLI the 460. AMD chipset don't natively support SLI.
New RAM and a new HDD aren't going to have any impact on gaming, but it's something you should do anyway, so I'm going to assume you do that regardless.
To try to sum up your options to see a large gaming performance increase:
#1: Upgrade your PSU and get a big new GPU.
#2: Replace current GPU with an ATI card, upgrade PSU and buy a second ATI card (assuming your motherboard supports Crossfire right now)
#3: Upgrade the CPU, PSU and GPU (AMD CPU)
#4: Upgrade the CPU, motherboard, PSU and GPU (Intel CPU, can either SLI the 460 or get a new GPU)
With #1 and #2, you're looking at a farily substantial upgrade, costing probably around $400-$500. You'll soon begin to be restricted by the CPU. With #3 and #4, you're basically building something new already, as that's the largest chunk of a build's budget. The AMD CPU upgrade option would eliminate the future upgrade needs of options #1 and #2, but would easily make the cost of the upgrade $600-650. At that price, you could be getting a completely new build with the i5-2500K.
What I would do if I had the budget you do is take the build I put together above. However, instead of buying the HD 6950, I'd buy a second GTX 460 and SLI them. That would save a good $150 or so, making it much easier to swallow. Also, if you have a decent case you currently like, you could leave out the HAF as well. If you go that route, you'd only be spending around $700 total.