Alright, I have a few questions, I'm not well versed when it comes to hardware specifics.
My current power supply is 300watts, would I have to upgrade this too? Sorry I forgot to mention this one in the initial post.
300 will be enough, but your next CPU/Video upgrade should include a new power supply for sure.
Will I need more cooling? I only have one fan, but I'm not sure how it will handle all of the newer things
Cooling should be fine as long as you're not overclocking, or already have a heating issue.
Do you absolutely have to upgrade to DDR2? It seems like a waste to get rid of 1gb+ ram like that.
To go to AM2 yea, you pretty much have to get DDR2, but if you got a 939 board you wouldn't be able to get a much higher x2 than the 4800, and no future quad core support. Also, you could get more than just 1gb to rationalize buying different ram. And then you would have a mobo+processor+video+ram to make an additional PC with, if you wanted. Maybe even sell it to a friend to make back some of that upgrade money.
I have 3 drives (1 optical, 2 hard drives) all on IDE, will the 4000 have enough for all of them? I'm not too keen on getting new hard drives as well.
Just about all motherboards today come with 2 IDE channels (2 devices per-channel) so it shouldn't be a problem.
I've never installed anything more complicated than a video card and ram, anything I should know before buying/installing?
Everything pretty much pops into place or screws in. The motherboard mounts to the case with screws, CPU is placed in the slot, and the heatsink placed and secured on top of it, power cords snap into the power plugs, IDE cables pop into their ports, RAM locks into the slots, connections from the front of the case go to the corresponding "jumpers" on the motherboard (tells you in the manual, if not on the mobo itself), and the power supply and PCIE/PCI cards slide in and are attached to the case with screws. You might have to reinstall windows, but that's all it takes!