I looked at Dell small business computers. You can get this:
Dimension 4300
1.6 GHz P4
256 MB SDRAM
Win XP home
20 GB HD
Total: $608 + shipping
If you upgrade that same thing to DDR it is $767 + shipping (Dimension 4400).
If you upgrade that same thing to RDRAM it is $777 + shipping (Dimension 8200).
Now for the fun part. You don't have to get a small business computer. The home Dimension 4400 is a better value. With the same things listed above, the price is $679 and free shipping! (possibly $709 and free shipping, I was confused if you can order without any speakers)
Sure there is a performance increase when changing from SDRAM to DDR or RDRAM (up to 30% faster). However, unless you play games, you will not notice the difference. Thus I wouldn't get the Dimension 8200.
The best value is with the Home Dimension 4400. You aren't going to get a much faster computer shipped for much less than $679. You could build an Athlon yourself for about $600, but then what if something goes wrong? Also what if Microsoft decides to prosecute you for illegally installing an operating system on other computers? Ok it won't happen, but a legal computer for $679 is hard to beat.