If you insist on staying with the 450W SmartPower (I recommend replacing it first), then don't chance it: get the 5770. It'll still be a big upgrade in performance with the smallest risk of something going wrong thanks to the PSU.
However, I am still worried that the PSU may be a low quality unit. Unfortunately, there's a risk of a PSU failure with lower quality units which has a history of killing one or more of your computer parts (sometimes killing the entire system). Higher quality units are much safer because if they don't have enough power, then they just simply turn off and that's it. However, lower quality PSUs will turn off too, except they may also die in the process which is when anything connected to it could be killed at the same time. So the reason why a lower quality unit would turn off in such an event is because it died.
As you may be gathering from all of this, the power supply is the most important part in a modern system (yours is modern in this sense). If a lower quality unit can't cope with the stress of powering a computer system - especially when gaming - then the result could be PSU failure along with the death of one or more of your computer parts. I fear that the SmartPower may be a junker.
Earlier, you asked me how I know that it's probably nothing more than a modern-day 300-350W power supply. Before I get into it, it's important to know that modern systems (including yours) get most of their power from the +12V. The spec sheet
says that the combined MAXIMUM capacity between the +5V, +12V1, 12V2 and +3.3V is only 410W. However, it also says that the combined output between just the +5V and +3.3V is 150W. So subtracting 150W from 410W leaves 260W on the +12V. Except, the first total is the "max" total while the 2nd one (the 150W total) is just the combined output - which almost sounds like a continuous 150W. Maybe. So I rounded up the possible +12V capacity to 300-325W which would mean that this PSU is just a basic 350W power supply at best (it's most definitely a 450W peak-rated unit).
Plus, the efficiency looks terrible too: it says that it's greater than or equal to 70% (it uses the "greater than or equal to" symbol), and this is usually a characteristic of a low quality PSU (or perhaps a much older unit).
Another thing I don't like is its MTBF: 80,000 hours at only 25Â°C. Now, 25Â°C is just 77Â°F which is pretty cold for a power supply! So that's like saying that as long as the PSU doesn't get used for more than just basic office applications, it should last a long time.
So I guess it's probably obvious by now that I would rather see you upgrade to a higher quality power supply before you upgrade to a better graphics card. It doesn't have to be some expensive 650W unit either! I mean even the 6850's power supply requirements say that a 500W peak-rated power supply is recommended for a system with a single 6850 in it! So that would mean that even a quality 450W power supply would be more than enough (meaning, not the 450W SmartPower).
You can take your maximum budget of $200 and spend about $70-$100 of it on a power supply, and then save up some more to get a really nice graphics card sometime after that. Try to think of it as an investment in some good protection for your system; after all, everything completely depends on the power supply.Edited by TwoCables - 4/30/11 at 8:24pm