Upgrading HP pavilion a257c

Hi,
I have an old HP Pavilion a257c that I use solely for gaming and it sorely needs an upgrade. If possible, I'd like to upgrade to a dual core processor, 1-2GB RAM, and a decent graphics card. My problem is that I'm not sure about compability with the current motherboard (I doubt it will work with the processor or the graphics card) and I'm not sure what the best upgrade options would be. I've looked through the forums and reviews, but honestly I'm overwhelmed with options. I'm looking to spend $200-400, much more than that and I might as well build a new system. If I have to buy a new MB, would it be better to just build a new system?

I'd love any advice in regards to components or just building a new machine from whatever I can salvage from this HP.


Thanks.