Yep the i3/i5s will probably need half the juice of an AM3 system....
I'm not sure really how you figure that one, but AMD systems don't need nearly as much juice as you seem to think.
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In choosing between your two choices I'd end up with the PII 720 over the i3 at this time. I'm not sure what's up with the newegg prices this weekend, but the price for the 720 is listed at $155 which is $35 more than it was just a short time ago...I'm sure it'll go right back in the next day or so. The normal price ranges on newegg from $119-124.00 with the stock heatsink assembly.
I did a quick look over and here's what I came up with...
Phenom II 720 cpu (OEM shown since the Retail price isn't listed correctly on newegg)
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103652
ASUS M4A78T-E motherboard (still has a rebate offer until the 14th I think)
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131366
G.SKILL Eco ram 1600Mhz cas7 1.35v
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231321
Xigamatek HDT S-964 heatsink (if you bought the OEM model PII 720 and need a heatsink)
http://www.newegg.com/Product/Product.aspx?Item=N82E16835233014
All that comes to a grand total of only $380.09 including shipping charges, but not counting the $15 you get back with rebates. That's a very sweet build I think, and has a lot of flexibility whether you want to overclock or undervolt the system. It's easy on the psu even at stock settings, and can be even better if you turn down the voltages manually (not using Cool 'N' Quiet).
Every Phenom I've ever owned or used (a lot) has been able to turn down the voltage significantly which saves loads of heat, stress, and power which is great. The stock voltages still aren't high number, but are there for insurance on AMD's part. Looking over a review of the i5's performance I see that at stock it needs roughly 1.25v @ 2.66Ghz and 1.55v @ 4.13Ghz. Those numbers are right in line with AMD processor results...and if you're going straight clock to clock in my experience the AMD can do the same on less voltage depending on the processor.
My PII 955 can do it's stock 3.2Ghz @ 1.168v, the PII 920 I had could do it's 2.8Ghz @ 1.20v, the PII 550 could do it's 3.1Ghz @ 1.15v (or better...never found the bottom before quitting testing). All of those do 4Ghz @ roughly 1.5-1.55v on air cooling so it's all the same. That's only a few of the Phenom's I've owned, and they can all run on very very low volts.
Power consumption is not in any way a problem with the AM3 platform, and remember the actual 790 chipsets suck down very low power as well (well known) so that's a bonus making the whole system use very little power.
Heck, my
total system wattage right now is only 112-116W while listening to music and typing this to you with 6 tabs and windows messenger running. That includes 5 case fans, 2 HDD's, 1 Blu-ray drive, 1 DVD/CD burner, 1 ATi 5770, 1 ASUS Xonar D1 soundcard, various usb devices, and of course the processor and motherboard. It's even better just sitting at idle lol. Did I mention the ram is sitting at very tight timings, the northbridge is overclocked, and the processor is NOT running any form of cool 'n' quiet or slower speeds on a QUAD CORE system?
Either way enjoy whichever you get.
Kei