Welcome, Alex!
Wanna check that model again? Gateway doesn't list a "FX2800".
If you meant SX2800 - here you go:
Computer memory upgrades for Gateway SX2800 Series (DDR3) Desktop/PC from Crucial.com
For name brand machines, we recommend buying ram direct from Crucial to guarantee compatibility. It's also recommended you install ram in pairs for dual channel operation.
At that link, a 2x2gb pair of DDR3-1333 is $29.99. RMA the module to Newegg.