Nearly all HP's use Micro ATX boards. A few use full ATX. And a few use full ATX cases with Micro ATX boards (yes, a smaller board does fit a larger case).
<A HREF="http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&cc=us&product=307863&dlc=en&docname=c00006476" target="_new">Here's your motherboard</A>. It's a Micro ATX version of the very well-liked Asus A7N8X. I'm not certain you can get a "much" better board, although you can probably get a more complete, newer version of the same board.
But you're also talking of a new CPU, so are you doing a socket change? Are you thinking of perhaps an Athlon 64?
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