Here's my reply to a similar post. Too lazy to make a custom response for every question. Note that my example system is more powerful than yours
Add 5w for each 120mm fan.
Does anyone have any suggestions on a nice PSU for about $150.00 - $200.00? It will be powering a AM2 X2 3800+, Asus M2n32-SLI Deluxe, 2 GB of RAM, and maybe two BFG 7900 GT's. If not two, only one video card. Your advice is much appreciated. I've heard that Antec work pretty well.
A 500w PSU is more than enough to power your system. I personally like the
Seasonic S12 500 PSU. - $130 Shipped
But you can also use any of the following PSUs:
Antec TruePower II TPII Blue 480w PSU. - $80 Shipped
ENERMAX Noisetaker EG495P-VE SFMA 485W Power Supply - $81 shipped after a $10 mail-in rebate.
FSP Group (Fortron Source) AX500-A - $96 shipped
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Any of the above is more than enough even for the following, more powerful system. These would represent power drawn from the 12v rails of the PSU:
Athlon X2 4800+.......................96w (an upgrade from the X2 3800+)
7900GTX #1.............................84w (an upgrade from the 7900GT)
7900GTX #2.............................84w (an upgrade from the 7900GT)
Typical Motherboard..................25w
Sound Card..............................10w
Typical 250GB HHD #1..............20w
Typical 250GB HHD #2..............20w
RAM..........................................0w (draws power from the 3.3v rail)
Total Power Consumption.........339w
Therefore, you will need a PSU with at least 30amps on the 12v rails (30amps x 12v = 360w).
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There is a CPU power consumption chart for the older S939 Athlon 64s. AM2 should consume less power though especially if you purchase one of those "power efficient" Athlon 64 X2 that have a TDP of 65w or 35w (AM2 X2 3800+ only).
Xbitlabs CPU Power Consumption Chart.
Here's a chart of
GPU Power Consumption.