Cores isn't the only thing. You also want to compare motherboard clock speeds and memory speeds. Sometimes that aspect of a board can bottleneck an otherwise faster CPU. Some boards also have the (stinky?) caveat where higher FSB speeds means less RAM support. I will say that going from 1 core (Athlon XP2900) to 4 cores (Phenom II X4 940) that the difference is night and day. An animation that used to take a week now only takes one day. Extra cores can also make the computer more responsive to multitasking. (But if you're rendering and using all cores with multithreading - it can still bog down.)

AMD vs Intel? Seems Intel still takes the top in overall performance. However AMD does seem to offer best bang for the buck regarding initial cost. (An AMD comparable to an Intel chip is priced closer to the next lower tier Intel model. AMDs are a little more power hungry though, and do run hotter.) Also since AMD owns ATI now, boards that support them are likely to have a better graphics chipset. Intel chipset graphics, from what I understand are still in the "meh" category. However if you plan on using a graphics card anyways, then that really shouldn't be an issue either.

Building it yourself, you can honestly get a nice fast computer for under $500. (Provided you keep the monitor, case, etc., from the previous one. Going from 2 to 4 cores, you'll probably also want a power supply that's around 600w or more, because 4 cores double up the 12v connection to the CPU. Look for EPS or 2xATX, which are pretty much the same thing.) Also on XP 32-bit, it's fine to stick with 4GB. Just note that a good portion of one gig goes to the OS - but unless you're doing something extreme, that's plenty of wiggle room. If you need more than that, there's 64-bit OS and software - then you can go crazy with RAM. But that can get expensive. (More on the software side than anything else.)

And yeah, can you tell I just threw together the new computer guts recently?

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.