The generously donated RAM worked. (Lord, am I relieved about that!). The slipstream install however was an "experience" and didn't go at all smoothly.
Firstly the partitions refused to format with the slipstream, so I decided to install 98 and then put 2000 on top.
Then there was a read error from the 98 disc. In the end I had to use my 98 CD to verify that I was upgrading to 2000. On the first attempt I got loads of errors about unfound files during the final setup. Second time lucky however, I repeated the proceedure again and it installed without a hitch.
Even with SP4 pre-installed it still threw up patches within minutes of connecting.
The process has taken all night and I'm not even finished yet, but at least I'm getting there!

Note to PC shop: I'll pass on those 512MB sticks at £100 a pop, thanks all the same!