Simon,
As JSessler said, match up your interfaces. This is important because of the wavelengths used for transmission. SX GBICs use 850nm, LX/LH GBICs use 1300nm. So the Cisco LX/LH will NOT talk to the 3COM SX -- incompatible wavelengths.
The wavelengths interact with fiber optic cable types in different ways, resulting in different maximum transmission distances. So it's important to know what kind of fiber you have, too. SX optics are rated for multimode fiber only (either 62.5/125 or 50/125-micron) and are limited to 220-275m runs.
LX/LH optics can also use MMF but need mode conditioning patch cables at both ends of fiber runs that are over 250m and up to to 550m in length. LX/LH normally use SMF and the 1000BASE-LX standard specifies out to 5km, but most manufacturers rate their optics out to 10km.
If you must use a Cisco LX/LH GBIC over appropriate fiber as noted above to a 3Com SX port, you will need a media converter at the 3Com side to take the signal from 1300nm to 850nm. Transition Networks (www.transition.com) is one company that makes these. Or, you could get a 3Com LX GBIC: CB9400 had 12 fixed SX ports and slots for 12 GBICs; if you still have a slot available, that's the way to go.
For the 2950G-48EI that will run two SX GBICs (trunked) to the CB9400, make sure the Cisco switch is set up to do LACP-based 802.3ad link aggregation and not Cisco's proprietary EtherChanneling.
Also, there's a negotiation setting that you may need to disable on the Cisco Gigabit port, to allow it to communicate properly with the 3Com. I don't remember what it is at this moment, it's been a while since I had to do this; but I do remember I had to do something. Some others have encountered similar problems with Dell switches and have posted here. Search here for "Dell" should help you find it.
Hope this helps.
