A low cost method to perform Eye-Pattern Testing on 100 MBit and 1 Gigabit Ethernet Transceivers
Both 100 Mbps and gigabit ethernet over twisted pair require about the same amount of bandwith, so a 500 MHz scope should be good enough to get an eye diagram for both standards.  However, if you are doing gigabit over fiber or SGMII, then this is not the case and you will need a faster scope.  

Both 100 Mbps ethernet and gigabit ethernet run at a symbol rate of 125 MHz, so most of the signal is primarily from DC to 62.5 MHz.  A 500 MHz scope provides 8x that bandwidth, so you should get a very good picture.  

One thing you may want to invest in, however, is a good quality active differential probe so you can make accurate measurements.  It is possible to subtract two channels with standard scope probes, but if they aren't exactly matched, you will get common mode to differential gain that can distort your eye measurement.