My wife uses 66x for her readyboost SD card Kingston "elite", and I'm not sure what few other people have for their speed, but they are definitely using the regular Kingston sd card, which might be 33x, but 2GB is like $7. The new SDHC (high capacity) cards' speeds are broken down as 2MB/s, 4MB/s, and 6MB/s (class 2, 4, or 6, not the 66x or 133x, which is multiples of 150KB/s (i.e. 1x = 150 KB/s), which was the first cdrom read/write speed (you know, then they came out with a 2x, then 4x, then 8x, then exploded to like 55x), just because people had gotten used to that x rating system. WAIT: so then we wonder, 133x (like Kingston's "elite") is 20MB/s (133 x 150 % 1000) is like 3 times faster than class 6 (or 6MB/s)?! This is because the new standard is a guaranteed minimum and the old 133x is like a theoretical max; Anyway, NOTE: SDHC readers can use SD cards, but regular SD readers do *not* handle SDHC cards.
Hopefully you can find out if your laptop (or desktop) has an SDHC card reader; otherwise, play it safe and get a 4GB SD card (the max size before having to get SDHC), or just get a 2GB SD card (which is a lot easier to find). If you do have the high capacity reader, then get what is being made (early 2010), which is usually cheapest (economics term: "technology frontier"): 8GB micro; i.e. albeit for mobile phones, just leave it in the regular SD adapter; e.g. by Transcend.
NOTE: READYBOOST has a 4GB limit (i.e. won't be able to handle 8GB), so either buy a cheaper 4GB (which isn't really the case) or just have 4GB to put some files on (you know, save your valuable data in two places); but, don't feel like you need more, especially since READYBOOST isn't adding more memory but rather turning on a more efficient disk access; i.e. you have it or you don't, not that you can have more.