yeah the simple best choice is buy another base unit.
If you buy a new motherboard and CPU then you will probably find that your hard drive and cd drives also need replacing as they will probably have IDE connectors and the new board will have SATA connectors
And even if you could reuse your hard drive windows wouldn't work after putting in a new motherboard and would need to be reinstalled.
Good chance you will need to replace the graphics card to as it will be a different slot.
Maybe the PSU too
In short as I said it's very very rarely worth upgrading a motherboard and CPU.... it works out cheaper to buy a new base unit