Don't forget to upgrade your RAM to at least 4GB...
The Dimension 3000 only supports 2GB max (2x1GB DDR SDRAM).
And, 2gb is enough for Windows 7. That's how much my HP Probook came stock with.
It was my understanding that when Windows 7 first came out, Microsoft basically said you could not upgrade from XP to 7 due to so many hardware compatibility issues. It may be time to recycle that old dell.
...is enough to work. It is not, IMHO, enough. My Dell 6000 laptop came with 512MB memory... it worked, but it was not optimal, even with XP. If the OP is going through the trouble to upgrade is OS on that old of a system, my thought is he needs to max out the RAM if he has any hopes of decent function with W7. TBH, I'm scared to try W7 on my Dimension 5150 because of the drivers...