The PC800 is a Rambus type chip. It has better memory bandwidth than the DDR does. PC1066 just came out, but it's harder to come by. The only company making it right now is Kingston. If you go the DDR route, the best you can get is DDR333. Unfortunately, Intel doesn't have any chipset that can take advantage of the full 333x2 memory bandwidth. If you decide to overclock you will be able to use it though. Try and find memory with the lowest latency possible. CAS 2.5 is average, but if you can find 2.0 then you will notice a definite difference. The 850 chipset is the Intel chipset that uses the Rambus memory chips, and the 845 chipset either uses DDR or SDRAM. DON'T get SDRAM for your P4. If you don't presently have a P4 and have to buy that as well, be sure you get one of the Northwood P4's. They are designated by an "A" at the end of their name. For a budget, the 1.6A P4 is a great chip to get because on the average you can overclock it to 2.4 Ghz. with stock air cooling.
As far as video cards go, all I can say is don't get any MX series card. They all pretty much blow. The GF3 Ti series is good, but if I were you I'd go with the GF4 Ti 4200 for the cost. Either that or the ATI Radeon 8500. They are both significantly more advanced and are pretty inexpensive. You can find the 8500 for around $100.00 and the 4200 for under $150.00.
Honestly if I were you I'd wait a couple of months. There is some really cool stuff coming out in the near future and it will significantly drop prices on the stuff you are currently looking at. It seems like you are an Intel fan, so that being the case, wait until the new 533 mhz bus architecture works its way down to some of the slower chips. Look for the "E" designation after the end of the chipset number. That will mean it supports the new bus speed. Also, we are on the verge of a whole new slew of graphics cards. We have the Nvidia NV 30 and NV 25 coming out soon, and we also have the ATI R300 and R250 coming out soon. Those will both dramaticall effect prices on current equipment and they will offer newer faster alternatives.
If you must buy now and you are on a budget, I'd get a P4 1.6A Northwood with an Abit or Asus MB running the 845E chipset. I would get at least 512 mb of DDR333 made by either Samsung, Corsair, or Crucial. For video I would go with the GF4 Ti 4200. Just do me a favor and DON'T get an MX card.
If you want the best prices on hardware you can get, check out
Newegg.com. They sell the new stuff for about as cheap as you can get it.