If you swapped out your power supply and now you're having trouble booting up, there's a good chance the PSU is the culprit rather than the graphics card. Whenever you see some ultra-cheap brand (I've never even heard of them) claiming an absurd amount of wattage, you know you're in trouble. A decent unit from a brand like Corsair or EVGA rated at 400W would likely outperform that junk Mentol unit claiming "720W" any day of the week.
As for what to do next, I'm stumped—maybe wait for someone with more expertise to chime in. Your best bet would be to borrow a higher-quality power supply to rule that out. If the PSU is indeed the issue, try returning it or trading it in for a better model if you just bought it. That 9800GT of yours pulls about 100W; it’s possible your current power supply is just too low-grade to handle the load. I have no idea what you could get for the card on the used market, though, maybe $100
That Nvidia GeForce GT 730 draws much less power, so it should work fine, but you really ought to ditch that sketchy power supply regardless. If it were me, I’d pick up a card at a local retail store. You can usually order from Best Buy even if it shows as out of stock, or just find it elsewhere.
http://www.amazon.com/details/vga-cardhttp://www.newegg.com/product/vga-card MSI offers a year's extra warranty compared to the other brands.
Looking at more powerful options, the Nvidia GeForce GT 730 fits your build quite well. Anything stronger than that wouldn't be worth the investment since you wouldn't be able to fully utilize its potential in modern games.