I'm in the market for creating a "newer computer". Ever since my college years I got into building my own PC's to suite my need, "my need for speed" :P. The creed I live by is really simple; Bigger, Better, Stronger (& everything that encompasses their meaning). It's something I picked up while working as a Fitness Professional (aka:Personal Trainer, but with an applied nutritional approach) and it was great because the facilities where open 24 Hours a day, :P, but I digress.
So the main point is teaching yourself about how to build a PC, because in the long run it's cheaper by only upgrading the components that need to be at the time of a malfunction, deficiency or wanting more UMPH as Madden says, lol. For instance, the current PC that I built has been in my hands for almost 4 years that has parts that go back to 5-6 years ago (the CPU) which happens to be an AMD 64-bit Processor 2.2 GHz equivalent to an Intel 3.5 at the time; then every so often better Drives, RAM, etc... Now with Windows 7 utilizing "Ready Boost" (using solid state drives like Flash Drives for extra RAM) life is even greater.
Basically heat is the main reason performance degrads over time (so clean the PC inside and out of dust along with reinstalling the OS every 18-24 months) besides the registries of an OS. Over the years the processing speed has not gone up much, but their "intelligence" has improved like the best one out there being the "i7" (Quad-Core with Core Virtualization meaning adding 4 more virtual Cores giving you a total of 8 Cores or a Dual Quad-Core) soon to be outdone by the "i9". Just keep in mind that on average every 18 months some new technology (CPU technology) comes out.
Now there are 2 types of i7 chips 1156 (8-series) & 1366 (9-series). The higher number basically indicates an average of a +200 dollar difference when you add up all the PC Parts like the Motherboard, RAM (Double or Triple Kits), CPU Fans, Power Supply (600-800 min. watts), PC Case, PCI Cards (video cards), Optical Drives, & Hard Drives.
So for the Power Supply requirements depend on how all of the parts produce power (watts). Like for every 128MB of RAM it needs about 7watts, cooling fans under 12watts?,CPU 130-200watts, Video Cards 15-30watts, etc... The higher the performance (more $$$) of the component the more wattage it needs.
I have a friend that just bought a desktop with an i7 860 with 6GB RAM and he wrapped up the performance with Revit with one word...."Wicked!". Now that Revit 2011 is utilizing multi-thread technology to an extent it is the best reason to upgrade to speed up the "BIM Process".
Investing now to upgrade your Home PC or better yet building one & using some components from your current PC is the best time unless it's been within a year then wait those 18 months before upgrading.