That depends entirely on your motherboard and PSU. If your motherboard has a second PCI Express slot, running at 8x or more (8x or 16x) and supports SLI, and if your PSU is powerfull enough (enough total watts and enough amps on the 12 rails), then yes you can SLI.
If not, then to SLI you will need to buy a new PSU and/or motherboard.
If that is the case, then just sell your 460 SE and buy a newer card, because a second 460 SE is going to be about £60 or something (if you can find one), but a suitable PSU is going to be another £60 at the least, and the motherboard could be £100 or more. And for that much money, you could just buy a new GPU.
The 460 SE is going to be OK with most games on medium settings or high. Some of the prettier games wont run on Very High settings too well.
If you upgraded to a GTX 660 or a Radeon 78XX or better, you would be able to run almost any game on very high settings.