Alien Hardware is expensive… if you’re a reasonably competent individual, however, you can do a great deal of the things yourself. Your case is probably a standard ATX case, so you can use your current case, power supply, and drives. I would strongly recommend that you replace your mainboard, processor, RAM and video card. Not a terribly difficult or technical process. The trick is, you want performance, value, and upgradability. If you’re an AMD fan, look for a mainboard with socket AM2, make sure it can support up to at least 4 GB of RAM, and has a couple of PCIe connections and an AGP connection. You’ll also probably want SATA capability, although that’s standard anymore.
More or less, what you want to do is get the best processor you can afford in the socket family for the board. AM2 processors include Athlon 64 processors up to the Athlon 64 FX series… so you can get something cheap, and upgrade to a premium level processor at a later point when you can afford it.
If you’re an Intel kinda guy, do the same thing, just find Socket T (aka 775). This runs from P4 processors on the low end to the ultra powerful Core 2 quad extremethingys that are on the cutting edge.
Just make sure whichever one you choose, you get a motherboard which specifically says it can handle the more powerful processors.
Know it’s a fair bit to chew on, but it’s not so difficult. Private message me if you’d like specifics about which motherboard/processor combos I’d recommend personally.