That Xilence unit is just some low-end Chinese office power supply. 😛 I doubt it even hits 400W, not to mention all the other areas where it falls short.
Since you didn't specify a budget, we just suggested something that matched the price-to-performance ratio of your other parts.
I was actually looking into the EVGA MaxPro series... it seems pretty solid, right up there with that beQuiet! System Power 7. That 600W model is $13 actually pricier than the beQuiet! 500W version, despite only gaining 72W in output. You could probably go with either one.
The NAXN and NAXN Advanced series are pretty low-end and honestly feel kind of cheap.
That Seasonic and the Corsair VX450 are almost identical in quality and look pretty similar, though I guess the Seasonic does have noticeably better efficiency.
Over at Best Buy, you can grab an MSI HD 7950 with about 20 months of warranty left for $417, which seems like a decent move... especially since the HD 7950 isn't exactly a power hog, so you probably wouldn't even need to swap out your power supply.
You might not actually need to swap out the motherboard; there are plenty of used CPUs out there that would be a massive upgrade for it.
You’ll likely need a new processor, a GPU, and a power supply. Just let me know what kind of budget you're looking at and where you're located... maybe we can find someone local to grab some parts for cash.