I'm afraid you're in for a shock. Without a monitor. A similar spec PC to yours, new, would cost somewhere round the £200-250 mark these days and it would come with Vista and probably another 512mb RAM.
Therefore I would say that you might get £100 for it.
Sorry but prices have dropped dramatically in the past couple of years, look at the prices at online retailers yourself.
You might be better either just keeping it and using it yourself or perhaps seeing if theres a worthy friend or relative that could use it.