sandisks are recommended mate, i use a 1gb sandisk and its fine, dont need anymore than a 1gb, the most you need it for is backing up your nand which is around 540mb iirc. you will get a 1gb from amazon or ebay for a few quid mate or any retail shop like comet, pc world!
you dont need to leave it in, once the hacks done you can take it out!