The ram's going to be £30 *max* second hand: £30 will buy you 2x1GB generic DDR2-800, £40 will buy you 2x1GB Kingston Hyper-X.
Splitting you might get more for the rig than £200, but only if you managed to realise top whack prices on everything. It'd be possible to build a similar spec rig for around £300 new, so I reckon £200 for the base unit is a reasonable offer.
As to the monitor, assuming it's in very good condition I'd guess around £60 wouldn't be unreasonable? It's hard to tell from ebay as none of the 20" samsungs on there have any bids yet...