I am thinking of replacing my tower, or finding a way of cheaply upgrading it from current spec. It is a basic PC tower with the following:

Asus MOBO

AMD 64 X2 4200 dual core processor

Nvidia GeForce 6100 nForce 405 graphics

1.75gb of RAM

I have installed a 2nd HDD too but due to power cabling options I had to kill my unused CR/DVD RW drive to power the hard drive. Hard disk space is not an issue though really as I don't take up masses of space on either drive.

and not much else. It generally runs OK but can be a bit slow and some games, not CPU intensive ones, can judder a bit.

Is it worth getting a replacement tower complete, or maybe spending £50 or so on upgrade parts for this one?

You will be limited to used in regards in CPU, unless your willing to replace motherboard. Might be the same with graphics as well.

I personally would go new, as there is not much in it in terms of cost (build v built)

Go new Jason, if you go secondhand you may well end up with another PC of simillar spec and end up spending more cash to upgrade it again.

The spec of your current PC is woefully out of date, and I can see why its quiet slow to use.