questionsis there anything i should be wary of while…

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You should be fine as long as you get a modern psu unit. Also, most modern gpu's require external power, and the cheaper ones may not come with that option. I also suggest (If you're willing to put out the extra cash) a modular unit. This will allow you to greatly reduce the amount of clutter in your box as it allows you to only use the cords you need.

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Since you're upgrading/replacing the PSU, take a look at the power draw of the new graphics card and make sure you've accounted for it when sizing the new unit.

I upgraded from a dual 7900GT (SLI) configuration to a single GTX 275 a while back. My computer had been randomly crashing under graphics load like playing games or running benchmarks. I thought I had a bad stick of RAM or bad graphics card, as my PSU was a good quality 500W unit, but apparently it lost some of it's zing as it aged (don't we all...). I was surprised to see that the GTX 275 drew over 230 Watts under load - more than double the old pair of 7900's... I replaced the 500 Watt unit with a 750 Watt supply and have been running fine ever since.

If you stick with your plan of using a 5450 you'll be fine, though, I just looked it up and it only draws like 16 Watts under load. If you decide to go bigger, check this out:

http://www.geeks3d.com/20100226/the-real-power-consumption-of-73-graphics-cards/