The only thing is, DDR266 isn't fast. So if your looking for performance, don't expect much. Espcially because that DDR400 will back clock to the speed of the DDR266.
Edit: Sorry just realized you are in the United Kingdom. You may want to buy from someone else other than www.NewEgg.com if shipping costs are too high.
It will just backclock to the slowest stick in your case DDR266/PC2100. The 4600 will run DDR400 but you would have to drop the DDR266 stick, you could get 2 sticks of DDR 400 or just get 1 larger stick of DDR400 and drop the 266.