Will a ASUS Geforce GTX 550 Ti work in dell optiplex gx280 small deaktop?

No the power supply cant handle it
the GX 280 if I remember right came with a 250w PSU
no more than a 300W
which would not be enough for the 550Ti
also the card would be really bottlenecked by your CPU
(high end Pentium 4-maybe Pentium D)

The card I would recommend for the GX 280
would be a HD 4650, HD 4670 or HD 5570 or HD 5670

but if you are looking to do hardcore gaming
then I am sorry to say you seriously have to think
about a new system

the GX280 is still okay for browsing,email,cd/dvd ripping
and office work but not a gaming machine