Sounds as if this thread is solved but I'm throwing in my two cents anyway:
A 460 watt ATX CoolerMaster is a high-quality unit that I'd absolutely keep a hold of – your system specs make it sound like you have a recent, more than adequate set of hardware components in there: plenty of physical RAM @ six gb and a perfectly decent main CPU on the motherboard.
Dell used to make their PC's so that only a Dell PSU would work in there but stopped playing that rip-off game years ago. But it's always good to double check..
Your 460 watt PSU will easily run an EVGA 520 graphics card and unless you feel an urgent need to upgrade the power supply to a higher wattage, will also save you money on your electric bill. Just remember to get in there and blow out all the dust and crap and keep it clean. That’s one of the best things to do for the longevity of your expensive PC components. I like to stick my hand behind my computer – palm towards the fans about an inch or so away – to feel for the temperature of the air blowing out of the back – it's always cool, almost cold except on the hottest summer days – lol. I have a Seasonic 350 watt PSU that has lots of 'headroom' for running my system - the reason for me getting a new card is because my 2 GB
of RAM and the CPU are being over-taxed by a new install of Vista home premium. I had previously run XP pro for 7 years.
So, all in all, my point is - if you’re set on the 520 graphics card you don’t need to spend any more cash other than on the card. I know cause I just went through all this myself with equipment very similar to yours – and guess what card I bought?
The 520.
Added: And if you have any installation problems with the card or anything else - I'm sure the good folks here on vistax64 will be able to steer you in the right direction - I can be very handy myself - and I'll let you all know how the install of the graphics card goes. Thanks!