They do have 1 gig kits available cheaper with better timings by the same manufacturer, but the 2 gig kit I see there that you listed would be the better value and give you the best overall perfomance if you have the money.
Either way performance wont be a big deal with cas 2 or 3 maybee 1-2%
You will need to get two stick of what you chose though to enable dual channel.
2x 512mb = 1Gb Dual channel
2x 1024mb = 2Gb Dual channel
1x 1024mb = 1Gb slower.
can i buy the 1 gig now then another 1 gig later to enable dual channel because i dont have the money right now, would that be the same as buying the 2 gig set or would the be mismathced, i dont mind not saving $50 or so cause just want to purchase asap.
If your mobo has four dimm slots for memory then yes you could do that. Just buy the 1gig kit (2x512) which would also allow you to run in dual channel and buy another of the same set a little later if you choose to do so.