Bit late I know (new to SB and the forum!) but I recently looked around for some new memory for a few 550s - I wanted 4x 1GB sticks. Some sites I found only had 512MB, others were quite expensive for 1GB. But, I did find these guys...
http://www.aememory.co.uk/Sun-Cobalt-Ra ... emory.html
They have 1GB sticks for under £40 (plus VAT but free shipping). I ordered, the sticks turned up a few days later and I've installed two in one of my 550s - all fine after a few days of operation.
They're Viking brand, code MD128722R8SN3-CL3, PC133R-333-542-Z (taken from one of the sticks). A bit 'thicker' than the 512MB sticks they've replaced but they fit ok and as I say, seem to be working just fine.
Hope that helps someone.