There is only so much that you can get for £600. Do you have a AMD 64 3000 right now?
You want to get pairs of memory for dual channel. When you go over 2gb's you start to bog down the memory controller so it will not be so much of a gain. By the time you need more than 2gb's you would want to get a new processor and DDR2 or what ever ram is around when you upgrade.
The CPU is always the bottleneck, so it wouldn't be overkill.
Have a look at some benchtests at www.tomshardware.com and http://www.anandtech.com/
You want to get pairs of memory for dual channel. When you go over 2gb's you start to bog down the memory controller so it will not be so much of a gain. By the time you need more than 2gb's you would want to get a new processor and DDR2 or what ever ram is around when you upgrade.
The CPU is always the bottleneck, so it wouldn't be overkill.
Have a look at some benchtests at www.tomshardware.com and http://www.anandtech.com/