I bought a 2Gb sony branded card from Ebay - UK seller. Cost me £32.00 delivered. However I'm experiencing problems getting it to work with my K750i...tried the card in a mates K750i with the same results, and tried his 64Mb MS Duo (note not 'pro') in my phone and that worked fine.
Issue definately seems to be with the memory card, just wondering if there's a limit on the size of the card that the phone will recognise? The card seems to function normally when in a card reader (can browse, download/upload files to it no problem).
Don't fancy asking for a replacement from the seller if the same thing is going to happen, just wondered if you have any suggestions - the phone is brand new and has the latest software update installed.