My PC has been running a bit slow recently so I thought about upgrading the RAM.

And when I saw this Corsair CMX4GX3M1A1333C9 XMS3 Desktop Memory 4GB on Amazon for £15.50 I bought it without a second thought. It arrived through the post today but I haven't installed it, this is mainly due to the fact that this RAM is DDR3 and reading through the manual for my Motherboard that I find that it is only DDR2 so I can't even use it!

And when I saw this Corsair CMX4GX3M1A1333C9 XMS3 Desktop Memory 4GB on Amazon for £15.50 I bought it without a second thought. It arrived through the post today but I haven't installed it, this is mainly due to the fact that this RAM is DDR3 and reading through the manual for my Motherboard that I find that it is only DDR2 so I can't even use it!

Comments
Find out what your memory usage is if you've not already.
Well whatever it's a fair guess, but try some diagnostic work. Also find out the FSB of your board and get ram to match, DDR2 comes in 664, 800 and 1066 mhz flavours. Get the fastest your board can handle.
Beware 1066mhz is also available in DDR3 too now, which it didn't used to be. Double check it.
System performance is many things.
Your PC might take an ice age to load, but games still run like greased lightning. You've got too much crap clogging your memory, you need more ram or less programs.
Also could be due to a knackered disk, reduced read/write speed means programs load slower but once loaded they run fast.
Now if your computer loads OK or not, but games run like shit also your CPU is too slow, it's overheating and slowing down or you've too many programs using CPU time.
Try a few things.
I haven't really played many games on the PC as I don't have a graphics card, the main problem is when colouring A4 illustrations at 300dpi as I end up with a large number of layers.
Opening the Task Manager and the CPU usage is generally between 2 & 5% but opening a colour file and performing a few operations and the usage goes no higher than 22%. The problem could be the hard Disks which are still IDE as I've been meaning to get a SATA II drive to replace the boot disk which should improve the read/write speed.
And thanks for your help!