The Variable speeds mean that your board will support all those speeds, the higher the faster (its just 8 x the clock speed of the ram), therefore pc4000 is 500mhz, pc 3200 is 400mhz, and pc2700 is 333mhz.
Just checked your board out, I think the info you quoted comes from the "crucial" test site.
You missed the part where it tells you whats in at the moment, scan again, and look under the diagram of each slot (top left of page). Slots 3 and 4 will say empty, and under 1 and 2 it will say either pc4000, 3200 or 2700. Both will be the same, and since your motherboard is dual channel, you need to upgrade in identical pairs.
If you are mega rich, you could go crazy and actually buy 4 sticks of 1gb as your board supports that much, but I can't think why anyone would ever need that much RAM (at present that is) for a pc that isn't acting as a server. Buy 2 matching sticks of 512mb RAM, so that all 4 are rated the same speed (4000,3200, or 2700) because if they are different, then the RAM will only operate at the lowest rated speed.
2 x 512mb pc4000 is $182 on that site and 2 x 512 pc3200 is $139, which I'm betting is what you have in your board now...you can probably cut quite a bit of that price if you shop around.
Hopefully you're sorted now