Depends on the ambient temp....remember the water can go no cooler than air and usually (with a good setup) you will hit 7C above air temp. i.e. Air temp 21C (average) +7C = 28C water temp = ~29C CPU temp idle. The load is where you really gain though as this would probably load at about 35C or so (depending on clocks and volts).
For £100 I wouldn't bother personally. You need about £150 to actually make a decent improvement on air cooling...
Then again, if you can pick up some really cheap parts... it might be worth it.
e.g. single heatercore = £20
Laing DDC (10W) = £35
Swiftech Apogee/MCW6002 = £25-30
Tubing + fittings (T piece + barbs) + additive = £15
So on second thoughts, it can be done....