hi chris
silent graphic cards are becoming more popular as pc builders are tired of mega loud graphic cards that use loads of power and require 600+ watt psu,s
however the main problem with fanless graphic cards is the limited speed it can run at without getting too hot
there are however a few decent cards around such as the Asus Extreme 8600GT 512MB Silent ( http://www.aria.co.uk/Products/Components/Graphics+Cards/PCI-E/nVIDIA+GeForce+8+Series/Asus+Extreme+8600GT+512MB+Silent+PCI-E+?productId=28044 ) which has a fanless design but still manages to have some pretty decent specs including a 540mhz core clock and 512mb of ddr3 memory running at 1400mhz,and with a 128bit memory bus and support for direct x 10 and pixel and shader model 4.0 this is quite a performer
another card worth considering is the Asus ATi Radeon HD 3650 512MB ( http://www.aria.co.uk/Products/Components/Graphics+Cards/PCI-E/ATI+Radeon+3000+Series/Asus+ATi+Radeon+HD+3650+512MB+PCI-E+Silent+?productId=31318 ) which comes with a ASUS V-Cool® Silent Heatsink for near silent operation
it has a excellent 725mhz core clock and 512mb of gddr3 memory running at 1400mhz,this is the same as the 8600gt and is around as fast as a memory clock will run without further cooling such as a fan or water cooling
for a budget card theres plenty of radeon HD3450,s such as the Club 3D ATI Radeon HD 3450 Passive Dual Heatpipe 256MB ( http://www.aria.co.uk/Products/Components/Graphics+Cards/PCI-E/ATI+Radeon+3000+Series/Club+3D+ATI+Radeon+HD+3450+Passive+Dual+Heatpipe+256MB+PCI-E2.0+?productId=33753 ) and the Gigabyte HD 3450 256MB PCI-E ( http://www.aria.co.uk/Products/Components/Graphics+Cards/PCI-E/ATI+Radeon+3000+Series/Gigabyte+HD+3450+256MB+PCI-E+?productId=33759 ) and both these cards have modest specs including a 600mhz core clock and 800mhz memory clock,and again the core clock is pretty decent considering its fanless design,however a.t.i have a knack of producing some excellent graphic card designs using fanless configurations
however most cards will require a power supply upgrade,there are little if any pci-express cards(and agp) that will run with a 240watt psu,and i am pretty suprised that you managed to run the 6600gt with less than a 350watt psu
a inexpensive psu upgrade would be ideal and a excellent choice would be something like the Xilence 480W Gaming PSU ( http://www.aria.co.uk/Products/Components/Power+Supplies/450w+to+499w/Xilence+480W+Gaming+PSU+?productId=19869 ) ,at only £22 this is a excellent and efficent psu
even something like the " 600W ATX SurgeControl Power Supply " ( http://www.aria.co.uk/Products/Components/Power+Supplies/500w+%2B/600W+ATX+SurgeControl+Power+Supply?productId=22653 ) would be ok,i recently bought this to accompany a radeon HD2600xt i bought for my wifes acer t671 and it runs fine,as a budget psu i would not overload it,however a single graphics card,2 hard drives and 2 x optical drives would be fine using this psu
finally if you experience alot of noise from your graphic card you can allways change the cards stock cooler,for example replacing it with a " Zalman VF1000 LED Silent VGA Cooler " ( http://www.aria.co.uk/Products/Components/Fans%2C+Heatsinks%2C+Coolers/Graphics+Card+Coolers/Zalman+VF1000+LED+Silent+VGA+Cooler?productId=32961 ) would reduce the noise quite significantly and as with all zalman coolers they are of excellent quality
any problems let me know
good luck mate !