"The Radeon HD 4770 is nearly as fast as dual HD 4670s in CrossFire mode, and is much more energy efficient than the GeForce 9800 GT"
This is a quote from a tomhardware article.
The price of a 4770 is about $102. It's faster and reccomended my Tomshardware. It will not cause any conflicts with your chipset. They may be from competing companies but they simply do thier job.
HD 4850 is a good buy, but he has a 400w generic power supply. Might/probably won't work out in the long run. If he can replace that it would be the logical choice. Otherwise, something low power like the 9600 GT, 9800 GT (energy efficient models for both) or HD 4670.
The 4850 will work fine on a 400w PSU given his system configuration. I've upgraded many computers from HP, Gateway, and Dell that have 350w and 400w PSUs with 4850s, and the PSU has never been an issue. They really exaggerate the recommendations on most cards because they have to plan for the worst case scenario of the system and PSU they will be used with.
+1 @jeffredo. We're not sure that's a decent 400W PSU (such as the ones Dell uses), it may be an utterly cheap generic, that will smoke if attached to a 4850. Without knowing more about it, the safest choices would be the low power nVidia 9800GT he linked, or an ATI HD4670.
Virajk, can you post what is on the label of your PSU? Brand and model would be nice, but at the least there should also be a table listing its outputs.
If the generic 400W PSU doesn't even have a PCIE power connector, then almost certainly its output won't be balanced for a modern system, with most of its load on +12V. You can use an adapter, but maybe not safely.
The HD4770, if you can find one, uses only a little more power than a 4670 even though it does need a PCIE power cable.