Looks like the video card came with the system. If your system is stable, and games are running smoothly on high settings, I would leave it alone. Dell has pretty good power supplies on it's high end systems. Only reason to upgrade the ps is if you overclock. I don't know if the board permits this. If you decide to upgrade the ps, corsair, pc power and cooling, antec, seasonic, enermax, and ocz are all good brands. Expect to spend about $100-150 for a 750w ps.
i've had my 435 for about a month. after constant lockups/freezing issues and hours on the phone with dell's useless tech support i thought i'd try a psu. it's run problem free for a week now. i got a thermaltake 850 watt from best buy for 160.
I'm running the same vid card, 8 gigs of ram and the 940 processor.