gt630 doesnt need power from the power supply. it runs from the power supplied on the pcie bus. Just about any power supply would run it but realistically you'd want at least 200w. Your 400w supply with 15a (15 x 12v = 180w) rail should be plenty to power a 65w card.
Edit - Pcie slot v1 thru 2.0 provides 75w of power : 3a on the 3.3v and 5.5a on the 12v. Pcie2.1 raises the bar to 150w.
Okay now another Issue....I know that PCI e x16 is backward compatible,but I have old Pci e x16 (v1,not v1.1) slot,now is THAT gonna be any problem??? cause I saw in the spec that it is PCI-E x 16 V2.0 (with v1.1 compatible) ???? will it work just fine on my PCI-E x16 v1 slot??
I bought a GT 430 to ensure slot compatibility (it is PCIe 2.0) and it works well, although I have also been told that you only need to restrict yourself to 2.0 cards if your slot is definitely PCIe 1.0.