MAC or PC?

Hi all,
I am in the process of looking to purchase a new desktop computer. I have already looked at Macs and a few Pc’s and was wondering which people thought would be the best in terms of value for money, performance and so on…
I already have an Apple powerbook laptop which iv had a couple of years. Its been great for what i want but i now need somehthing capable of handling lots of informations for music programs and video editing.
Do anyone have any recomended suggestions as to which spec to go for and where to buy from.
I have to say at the minute i am tempted to buy the new mac pro 8core machine 3ghz, but if anyone knows of any pc’s that can rival this for a better price, please let me know! Im not that clued up on technology so your suggestions will help no end!
Is there anything on a Mac that can not be upgraded after purchase? In particular the processor?