Phenom II X2 550 BE

[mikeymike]

DDR3 or DDR2 RAM?  AM3 is DDR3, AM2+ is DDR2.  AFAIK, DDR3 gives a steady speed increase from DDR2 the further up you go in FSB MHz, though each new generation uses less power than its predecessors.

PSUs - a cheapo 400W may not be able to do anywhere near 400W, at least not in a way that matters (eg. decent ampage on the +12V rails, or "it can do 400W for about 20 seconds before it blows!  great huh!").  I always go for Corsair VX450W PSUs - virtually silent, and will probably do unless you want a high-end gaming system with multiple optical and hard disks.  It comes with a 5 year warranty too.

64-bit OS?  If not, then >3GB RAM (plus a little change) is of no use.

You haven't made it clear whether you have a GeForce GT240 or if you're just planning on getting one.

[mikeymike]

I'm just waiting for the inevitable, where AMD overtakes intel again. It's like a 2 man relay race.

Bear in mind that the latter-day Pentium 4s (around the 3GHz mark, before and during prescott core) were a tactical disaster for Intel, and finally brought to an end the idea that the higher the GHz, the faster the processor.  I think it needs Intel to make a mistake of that magnitude again for them to lose the performance crown a second time.

The only other factor that might make a difference is how Intel's anti-competitive behaviour against AMD has been brought to an end, for now.  However, that end would have been to AMD's greatest advantage when the Athlon 64 was ruling the roost.

AMD buying ATi has proved to be an advantage for AMD since they started putting discrete graphics chipsets on-board, it helps them sell their wares as the value-for-money choice, but considering that the Intel Core 2 Quad still gives the best AMD has a run for their money, AMD has a lot of catching up to do.

[mikeymike]

I've built one system for a customer with a Phenom II X2 550 BE.  I wasn't particularly happy with the fact that the processor ran significantly hotter (~40C idle) than a Phenom II X4 945, a Phenom II X3 710 or any Athlon II X2 I've used (the rest about 30C idle).  In hindsight it seems almost unbelievable that the CPU fan span faster on that processor than any of the others I've used in system builds, yet the CPU temp was higher, but I definitely remember thinking "that's the last time I use that processor".  Every time I was using retail CPUs, which all have the same HSF.

Next time I'm with that customer's computer, I'll check the fan speed and temps.  Admittedly the fan speed wasn't high enough to be irritating (all the other systems run virtually silent, to the point that I can hear the booting beep echo inside the case).

I agree with koshman that pretty much any 400W PSU will be fine, though I tend to invest in the better ones because they're quieter, and because a failing PSU can sometimes take the board with it, and I believe a better quality PSU is less likely to fail.