I agree with
gg that eBay is the way to go - certainly a shed load cheaper than £150!
boguing also has a point as your system appears to use RDRAM & not SDRAM (commonly known as DDR) according to this :
Documentation
It would be worth opening the box to find out exactly what you have & how many modules.
Search on eBay for 1gb rdram. I think you need PC800-40 as the documentation mentions 40ns or better for the speed but I'd buy to match what you have.