Some component replacements: any issues?

[KNiVES]

My current system is decidedly extremely poor. The CPU won't overclock past 2.53ghz (to this day I have no idea why - I can't rule out the CPU, the motherboard, the PSU nor the memory), the memory is pretty poor and the power supply is showing its age. Doesn't help that the motherboard has incredibly retarded overclocking utilities and MSI's customer support is non-existent. They are basically holding my 4870 back. So to this end, I've decided to look at getting them replaced.

Any incompatibilities or such I may have overlooked here?

If there's any alternative approaches, I'd like to know too. Cheers.

(I already have the case, cd-drive, etc etc)

 

[KNiVES]

I had another look at my current memory, it's actually the Corsair DDR2 PC2-6400C4 4-4-4-12 (got 2x2GB of the stuff), is there any point in buying what I've chosen when I've already got similar?