Contact Gateway and ask them if it is possible to upgrade the motherboard - I don't think so.
What you can do is to add memory. This computer will handle 8GB RAM. Installed: 4096 MB DDR2, 800 MHz, (PC2-6400) dual channel memory (four 1024 MB modules)
Expandable to: 8 GB (4 DDR2 DIMM slots)
It comes with two 500 GB HDDs, and you can add one 1TB HDD in the 3rd. bay.
It comes with NVIDIA GeForce® 9800GT video card, 512 MB - you should be able to upgrade this to a 1GB GeForce Video card. NVIDIA will be able to guide you on this - several choices available.
The 700 Watts PSU is adequate.