Old Memory + New Mobo= okay?

*Newb Alert*

*Cuidado! Piso Mojado!*

Hey gang, I'm attempting to build my first PC and my first overclock here in a day or two when my parts arrive. And I have a few questions for you fine folks. They are:

1.) Will old pc2100 memory work on a new Gigabyte 1000pro mobo which uses pc3200? (I want to use a couple sticks of the pc2100 ram out of one of my other pcs for the time being until I can get some nice ram)

2.) I am tring to find a good price on a pair of Corsair XMS Speed series 512mb pc3200 RAM and while looking on Newegg there were two types:

pc-3200 C2 (2x32Mx64) w/heat spreader
-and-
pc-3200 C2PT (64Mx64) w/platinum-silver heat spreader OEM

What exactly is the 2x32Mx64 & 64Mx64 for? And which would be better for the giga mob and a p4 2.4C combination?
Both sticks were priced the same @$123.50. Anyone have a better/cheaper memory solution?

Almost forgot, One last question.
I read somewhere that it is better to go with a higher mhz ram if you plan on overclocking. So would it be better to buy say pc3500 RAM?

Thanks in advance for your help!

The pc2100 should be fine until you get new ram. I suggest corsair pc3200 or pc3500. For the most part, good pc3200 can reach high memory speeds.

The PC2100 should operate, although you will need to run the 5:4 or more likely the 3:2 memory ratios to stay within its lmits.

Don't get the corsair. It is CH5 chips these days and CH5 does not jive with the 865/875 chipset. Get the Buffalo 3700 BH5 for $99 per 512MB. If you must spend more, consider the Kingston HyperX 3000, it is also a BH5 variety. HyperX 3200 (non-A) is also BH5, but pricey. HyperX 3500 is the same CH5 found on the corsair, and inferior to the BH5 types.