Amperes (A) is the amount of power suplied by a PSU or needed by any part in your PC. Modern PC's draw most power from the 12V rails, and thus these are the most important.
The relationship between Volts (V), Ampere (A) and Watt (W) is easy: W = V x A.
In the example you supplied, the PSU has two 12V rails, denoted by 12V1 and 12V2. Each rail can supply 18A. However, the total output on the dual 12V rails is limited, often to an amount less than the sum of the dual 12V rails.
Some examples (see images below):
The ENERMAX Liberty ELT500AWT ATX12V 500W has dual 12V rails of 22A each. However, the total output on the 12V rails is limited to 32A (384W).
The Antec TRUEPOWERII TPII-550 ATX12V 550W has dual 12V rails of 19A each, but a total output on the 12V rails of 36A.
Thus, at first glance it seems that the Enermax has more power on the 12V rails (higher amps per rail), but when you look closer at the specs, the Antec can deliver more total power on the 12V rails.
Finally, take a look at http://www.slizone.com/object/slizon..._powersupplies
. You'll find a list of SLI certified PSU's. Go with any well known brand name. Both PSUs I meantioned are SLI certified, but if you plan to overclock, you should get something a bit heavier than the minimal requirements (i.e. I'd pick the Antec over the Enermax.)
Do your research, since there are other good PSUs out there..these two are just examples. Let us know what you pick, and we can advice you further.