PSU, Motherboard, and 12v Connection Question. . .Help

I am totally new at this, and confused. . .

I had posted previously that I was concerned that my Antec Smart Power II 400 W power supply was not going to be enough for my build. . .

Existing Antec TX640B w/SP400 watt SmartPowerII PSU
MB MSI K8N NEO4 Platinum NF4U 939 - Retail
EVGA GF 7600GT 256-P2-N553-AX - Retail
CPU AMD|ATHLON 64 X2 3800+ 939P R - Retail
2GB MEM 1Gx2|D400 OCZ4002048ELDCPE-K R - Retail
1 HD 250G|WD 7K 16M SATA WD2500YD % - OEM (but w/ 5yr warranty)
Existing PATA drive (I think a 250MB 7200 of some sort)
2x existing NEC 3550A DVD RW Drives
CPU THERMPASTE|ARCTIC SILVER5 3.5G%
2 EVERCOOL 80MM FILTER FAN SFF-8
1 EVERCOOL 92MM FILTER FAN SFF-9 RTL

Several replies said that it would be given that it was non-SLI.

I put the motherboard in the case last night and installed the CPU. I started to look at the plugs, and I have two of those 12v plugs from the Power supply. One actually says PCIe on it. The motherboard only has one plug for this, and the video card does not have any sort of connection for a plug. Do I just plug in the single connection to the motherboard and leave the other hanging?

Also, the max amps on the PSU is 15A on one of the 12v. My video card calls for 18A, but I truly believe that is only if you are running SLI. True or false?

Although I would rather not spend any more money, if I have to buy a new PSU, please suggest one for around $50 that would get the job done. Ideally, I would like to just go to CompUSA and pick it up. However, you will be my friend if you will say that I am fine the way I am and not to worry about it!