7dayshop are normally pretty good. I think for HD video you want a fast card
I would have guessed a 'minimum' of class 4.
Mymemory are doing a Transcend 8GB class 10 for £15.99 at the mo.
but why are other brands, lower class and/or lower memory more money than the transcend?
That depends on the camera. On the Nikons, with them only shooting 720p and using M-JPEG codec, you only need about a 3MB/sec transfer rate, so Class 4 will be fine for your D90 (I use a 16GB Class 4 in my D300s and it's never had a problem with stalling or skipping frames).
How fast you'll need for the TX5, I dunno. You'll have to work out the data rate (just record 30 seconds of video, and divide the total filesize by 30 to find out how many MB/sec it uses, and there's the speed you need).
thanks. hmm, could do with saving a few bob, but now tempted by the sandisk class 6 8GB @ £20
http://www.mymemory.co.uk/SDHC/SanDisk/Sandisk-8GB-Extreme-SD-Card-(SDHC)---Class-6