You say it's only 2 years old but is lagging. Personally I wouldn't have thought it was a hardware problem on a quad-core after only 2 years. Although I see it's overclocked so no doubt you have improved the fans and venting to allow for increased heat.
Other than dust around the fans and vents, I would then look at software itself. Even after 2 years a system gets clogged up with software installs affecting startup and load on the CPU and RAM with running Processes and Services, so may be you can trim those down.
As to hard drives, I don't think it really matters if it is for storage only. An internal will increase the load on the PSU but 750s watt should be fine. However I understand that PSU wattage drops over time, so you could check that. It's just as easy to copy off personal files onto an external, (and don't forget your backup copies on additional external media), as it is to an internal.
I'm no technician, but I'm guessing that cleaning the inside of the case and housekeeping the install itself would help a lot.
I've had my current desktop for about 2 years now. Back in its day it used to be a BEAST but I'm finding it lagging behind a little. I'm just going to post a list of my hardware and have you guys point out what you would think would be the highest priority upgrade. Thanks for all the help! (Sidenote: My HDD is running out of space, would you recommend just buying an external or getting a new internal and making it a slave drive?)