Advice on RAM for iMac, please

I currently have a 2009 iMac with 2GB RAM. I would like to install Aperture 3 but understand that it needs more memory to run well. Checked out the Apple Store and found that 8GB costs £320, so together with the cost of Aperture 3 from the Apps store, I would be spending about £400.

It isn't impossible, but it seems a lot. So I checked out the cost of RAM on amazon.co.uk and found much cheaper alternatives:

http://www.amazon.co.uk/memory-upgrades-4GBx2-1067Mhz-computer/dp/B004TQCK36/ref=sr_1_8?ie=UTF8&qid=1302788365&sr=8-8

My question is, does anyone know if these alternatives are ok?
Am I foolish to even consider them?
Is there any risk, either to my computer or my photos?

Any advice welcome please.