You're motherboard will be fine with a 9800GTX+. I don't foresee any issues, though you should probably update the BIOS to the latest version just in case. You can find that here along with instructions:
ASUSTeK Computer Inc.-Support-
And going by that Chinese website, the
ST50F (note the F) does NOT come with a 8pin to 4pin CPU power adapter, nor does it have a 4pin CPU power connector, only the 8pin. However it would appear that later models introduced a 4+4pin CPU power instead of having a solid 8pin, meaning that you can break away the un-needed additional 4pins. At least that's what Silverstone's website would leave me to believe. Newegg customer reviews said otherwise though.
However the
ST50EF (note the EF), states that it does come with an adapter:
SilverStone Technology Co., Ltd - Designing Inspiration Quote:
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1 x 8-pin EPS12V to 4-pin ATX12V adapter (150mm)
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And upon further inspection, the ST50EF (again noting the EF) has a much more robust design internally as well:
Compared to the ST50F (again, noting the F only)
You can see that the EF has more components and larger heatsinks which normally would suggest that it's a higher quality PSU. Another bonus is that most of the cables are sleeved on the EF as well so you'll have a cleaner look inside your case (if you have a side window). So I'd definitely go with the
ST50EF (note the EF), and it's actually cheaper as well as previously mentioned. Many reviews for the EF as well which all seem to say it's a great PSU. Even
JonnyGURU said so and he's pretty well known for doing great PSU reviews.