What are you comparing the cost of?If you already have a lot of SDRAM that you're thinking of putting in to your new system, then use that and save yourself some $$$.
If you have to get new memory for the system, then shell out the extra $$$ and get a DDR SDRAM system. DDR SDRAM isn't all that much more than SDRAM. Applications that have good 3DNow! optimization will show a pretty good performance increase. Insist on the AMD 760 chipset. The Via and ALi chipsets are a lot slower, and you won't save much $$$ by using them.
Side note : I've been playing around with MMX / SSE / 3DNow! instructions lately. On the Pentium IIs and IIIs, shuffling data around with MMX is no faster than using normal instructions. Using SSE cache prefetch gives you a tiny performance increase (about 3\%). On the Athlon, proper use of the 3DNow! cache prefetch instructions along with MMX results in 20-25\% more bandwidth 
