Difference betwteen 800MHZ and 1066MHZ Memory

How much difference does it really make if memory runs at 1066 MHz rather than 800 MHZ?

I'll be getting 3 or 4 GB of RAM.

Probably a 500GB 7200 rpm SATA 3Gb/s hard drive.

OS 64 bit -- Vista Home Premium now, 7 eventually.

Probably either an Intel Pentium Dual-Core processor E5300 [2.6GHz, 2MB L2, 800MHz FSB] or an an Intel Core 2 Quad processor Q8300 [2.5GHz, 4MB L2, 1333MHz FSB].

I am NOT a gamer but will tend to have several windows and apps open at same time, and would like to be able to run video off internet without problems.

The only difference is when you overclock, otherwise its pointless and usually more expensive for such a pathetic gain - ~3%

btw get a matched pair for dual channel -> 4gb (minimum)

I tried both frequencies and didn't see much difference on a C2D E8600 +P45. The Fsb limits the throughput anyway; high Ram frequencies reduce latency.

To run videos, you don't need a C2Q. To open several applications neither. And anyway, a quad-core on an Fsb is less than optimum: take either a C2 dual-core or an i7 / Amd quad-core. And since you won't use the four cores, at that price, take rather a faster dual-core! An E8400 (Fsb at 1333MHz) costs at much as a Q8300 and will run your single-task applications faster; prefer stepping E0 to reduce heating.

You didn't tell about the chipset, and though it's quite important! More than the Ram.

500GB disk: capacity per platter makes the throughput, and at 7200rpm, only Seagate's 7200.12 has 500GB in one platter. Very silent, but the arm is somewhat lame (nothing tragic). At any other manufacturer, this capacity would be suboptimal. For instance at Hitachi's 7k1000b (one of the last agile arm), good capacities are 320GB, 640GB and 1000GB to get the full 320GB per platter and hence the full throughput.