I'm looking to upgrade my Dell XPS 420 (yes, I have had this setup since 2008) and I've searched the forums and you've been very helpful! Main purpose of my system is normal pc stuff, plus some massive picture projects (Corel) and online gaming (lord of the rings online).
I am currently running Intel E8500, 4GB (4X1GB DDR2), Vista 32 bit, Radeon 3800 series, 375w power supply.
From all my reasearch, I've decided to upgrade to Win 7 home, change out my 4x1GB DDR2 for 4x2GB DDR2, and upgrade to the GeForce GTX650 (it's cheaper than the GTX 460). However, that means I need to upgrade my power supply to at least 500w, and I was contemplating a 600w, so that when I build my next PC (have to learn a lot more before I go down that path) I have enough power. But I'm worried my motherboard won't be able to handle a higher power supply. Thoughts?
Power supplies can run at below 100% load (in fact, they usually do), so you don't have to worry about them being too powerful for other components. It's the other way around - you need to make sure the power supply can provide enough for your components. You could technically run low-power components off a 1500W power supply, though that's not recommended for other reasons (price, efficiency drop at <20% load, and the physical size of the psu).
Actually, I've heard that some prebuilt computers use a non-standard power connector, which could lead to motherboard failure when used with a standard ATX power supply. That's separate from just having a higher wattage, though. I can't find a confirmation of whether that particular model of Dell uses a proprietary power connector or ATX power connectors.
Find the 20-pin or 24-pin power connector on the motherboard, and compare the wire colors and arrangement with the ATX standard: http://en.wikipedia.org/wiki/Atx#Power_supply
If you have a multimeter, you can also check the voltages.