upgraded motherbourd/ram to these

http://www.ebuyer.com/UK/product/124972

http://www.ebuyer.com/UK/product/91124/rb/0

and decided to throw its toys out the pram. installed windows and was comming up with errors and randomly rebooting underload.

Bro said it could be my psu, running a 400W one that came with the case i got years ago, it has 20 pin and 4 pin power connectors and the mbourds 24 and 8 but was hoping i'd get away with using the old one as the mbourd manual states a minimum of 300W but does state that i'll quote from the manual "The power supply with 20-pin or 24pin can both be connected to this 24 pin connector. However, a 20pin power supply may cause the system unstable or even unbootable for the sake of insufficient electricity..."

looking at this one to replace it
http://www.ebuyer.com/UK/product/128675

but i'm umming and aaahing about whether to replace it as he also mentioned the cpu might need changing to (more money!!)

i'm running a p4 3ghz fsb800 1mb cache, bit of an old girl but does the job and the spec list states its fully supported, the only reason i think it could be this is after the comp crashed it would come up with a cmos scan but at the bottom it said "cpu is unworkable"
but surely if the mbourd didn't support the cpu the comp wouldn't boot at all?

Bit of an essay i know but this has left me a little confused and a bit worried as i just wonna get back to owning on bf2

any help and advice would be appriciated