the equation is actually a lot more complex than the above posts suggest(not you coldfire, you put it properly), but such complexities will only confuse the opening poster
for example a 800Mhz ram can be sync'd to run at 667mhz (or any speed at or below 800), while running at a lower latency than RAM rated for 667mhz. so go ahead and buy a cheap pair of
4GB 800Mhz DDR2 ram
if you state your budget it is easier to recommend the parts, but heres a general idea of what you should go for:
4GB (2x 2GB sticks) DDR2 800Mhz RAM (dual channel helps with mamory bandwidth and latency a lot more than latencies or the ram being synced

)
Gigabyte P35-DS3L Motherboard
320GB seagate/maxtor/samsung HDD (7200rpm, preferably 16mb cache)
Tt 500W PSU (deluxe does the job too, but i prefer not to take the risk)
Fit the processor and graphics card accordingly within the budget (i.e faster processor and slower GPU for general tasks, faster GPU and slower processor for gaming)
note: try not to get more than 512MB framebuffer for graphics cards. it is a waste. Also for the GPU, try to alteast get a geforce 9600GT or faster card, preferably the 4850