I'm currently running a custom build (about a year old) and it seems the CPU temps are higher than in the past. The build is as follows:

Antec 300 case
1 140mm top case fan (run on low SPEED)
1 120mm back case fan (run on low speed)
AMD Phenom II 965 BE 3.4ghz (no OC, stock cooler)
12 GB RAM (using all RAM slots)
Gigabyte MA790GPT-UD3H
SAPPHIRE 100283L Radeon HD 5770
Corsair CMPSU-650TX 650W PSU
Samsung F3 1TB HD
WD 320GB HD
Windows 7 64-bit

I bench tested it a year ago before the video card addition (so integrated graphics) and got

NO VIDEO CARD

Idle: 31 C
Load (PRIME 95 for over 1 hour): 67 C

I just recently decided to test for temperatures again and used Prime95 with CoreTemp to monitor, and the temperatures were getting fairly high.

(WITH HD 5770)

Idle: 37-38 C
Load (Prime 95 after 2 minutes): 77 C

Video card Idle: 58 C

I actually stopped the Prime 95 test about a few minutes since the CPU temperature kept GOING up and I didn't want to hit the CPU Tj Max (78 C according to CoreTemp).

Should I be looking to add more case FANS to the front and side? Should I probably consider a new after-market CPU cooler like (http://www.newegg.com/Product/Product.aspx?Item=N82E16835233001)?

I'm concerned about the stability of my machine once I get to putting under more stress playing games and such. I'm looking to keep my computer as quiet as POSSIBLE (one reason I run the fans at low speed), but still maintain good cooling so stability isn't a problem.

Thanks for any suggestions.Did you modify the fans to run at lower speeds? Well if you did, that's whats causing your over heat problem . obviously when the heat comes up the fans are controlled by the motherboard to run faster. but if it's making a big noise you can try a silent cooling fan  , refer to the links below. There is also some tips for silent computer cooling. A quiet cooling fan is 12db or less, the lesser it's the quieter it's.

http://www.endpcnoise.com/cgi-bin/e/cpucomparison.html

http://www.endpcnoise.com/cgi-bin/e/computercooling.html