Yes, a RAM disk would provide you with an ultra fast tempdb, however there are very few databases which actually need this sort of speed boost. Purchasing enough RAM to be able to create a RAM disk large enough to fit your needs may get very expensive. Once you start needing to use the 4 Gig or 8 Gig memory sticks your costs start to go up very fast.
A Dedicated RAID 10 array with 4 drives is usually fast enough for most databases, and is usually a much less expensive solution.
No that’s not correct that SQL 2000 can only use 4 GB of RAM (you put 4 MB, I assume you meant Gigs). SQL 2000 standard can only use 2 Gigs of RAM. SQL 2000 Enterprise can use has much RAM as the host OS Supports. If you are using Windows 2003 Standard than you are limited to 4 Gigs of RAM. Windows 2003 Enterprise supports up to 64 GB of RAM.