hi i am thinking of buying a ds for my girlfriend what kind of memorycards do they take? Also do you need to put firmware on them like on a psp?

You need a linker, like the R4 or M3 simply to name popular ones, and a micro sd memory card. Everything is dead easy, and you do not risk to brick your console or anything.
She will be delighted :)

PS: a lot more of informations: http://www.ipmart-forum.com/showthread.php?t=162316

cheers m8 its lot of reading but sure i'll manage i had a look at the site where do you get the R4 tho it doesn't say on the site?

There are a lot of websites selling it online, it would get spammy if I write down any of them here...
You may find linkers in conventional gaming shops but it is usually more expensive