I plan on buying another 8800GTX soon and i have to buy a better power supply as the one i have now is not good enough

im currently using a coolermaster 550w power supply (only 2 pci-e i need 4 dam-it)
I'm thinking of buying the thermaltake tough power 850w with cable management
the person who built my pc did a really messy job with the wiring

iv never swapped a power supply before? but im confident i could do it though

my question is is it a simple job or should i get a professional to do it?