You won't see a difference in speed between a PCIe or PCI network card. Although PCIe is quicker than PCI, n networks only work at a maximum of 75MB/s (600 Mb/s), which is slower than any PCI lane runs at, so the card is the bottleneck, not the PCI lane.
As to the PSU, the calculators are sort of what I was talking about. They will say "you need a 700W PSU" or whatever, but not all 700W PSUs are the same. The reason some cost $20 and some cost $100 is because, not surprisingly, the more epensive one is a hell of alot better. It is more reliable, the power is cleaner, the unit is more efficient and generally you get better perks (warranty and what not) with higher quality units.
Unless you have already bought the unit, I would switch to something more reliable. It is a CWT unit, but not the greatest. Antec, Corsair, XFX, Silverstone, Seasonic, BFG, Pc Power & Cooling and Enermax are good choices