If you're buying a PC, pay attention to the type of memory it uses, because some memory can be far more expensive than others.
For example, http://www.pricewatch.com/ recently showed numerous sources offering 128MB PC100 SDRAM DIMMs for $106 plus shipping. For comparison, 128MB RAMBUS RDRAM memory was quoted at Pricewatch for $248 plus shipping--over twice the price of SDRAM!
If you are considering the more expensive memory because you believe it has crucial performance advantages, you may also want to see "Rambus Requiem--RDRAM Fails Bandwidth Tests" at http://www.tomshardware.com/mainboard/00q2/000529/index-26.html