I had a similar decision to make and decided a new computer was the best option. While upgrading memory, hard drives, and video cards will get you some nice gains, its still an older computer with a likelihood of another part failing (motherboard, power supply etc), then what? Assuming this is how you make your living, the prudent option is to plan on replacing your hardware every 3-4 years or so. You might be able to extend that with periodic, specific upgrades, but I wouldn't go past 5-6 years, I think you are asking for trouble if you do.