hi bob
nvidia based cards are good overclockers however its all down to the cards cooling set-up and in the case of the geforce 9500gt there are some decent vga coolers however there are also some bad vga coolers that wouldnt fair well when overclocking
in regards to overclocking there are three utilitys/programs that come to mind and these are :
1)N-TUNE : ( http://www.nvidia.com/object/ntune_5.05.47.00.html )this is a awesome nvidia utility and it makes overclocking avalible to even the most novice end user...you can overclock your cpu and gpu in safety and at no time you need to worry about damaging your graphic card or processor ............the overclocking can be done as a coarse overclock which takes around 20 minutes and will increase your clock speeds to the highest and most stable settings....and then theres the "fine" settings that take 3 hours
2) COOLBITS(http://downloads.guru3d.com/download.php?det=815 ) : this is a registry hack from nvidia and allows you to manually change your core and memory clock speeds...however this may not work on the latest forceware drivers,,,, however when it does work its a good little utility
3) RIVATUNER ( http://www.guru3d.com/index.php?page=rivatuner ) : another excellent tool and popular with other overclockers and enthusiasts....has a built in test to check for artifacts after applying changes
and as i have mentioned you may need to improve your cooling when overclocking and this does not have to break the bank,,,for example the "Akasa AK-VC03-BLUV Vortexx NEO VGA Cooler" from http://www.overclockers.co.uk/showproduct.php?prodid=HS-047-AK&groupid=701&catid=57&subcat=787 ...is a awesome vga cooler and fits on nearlly all nvidia graphic cards,,,
the aim of the exersize is to increase your gpu,s speed while keeping its core temp as low as possible....some enthusiasts use waterblocks as part of a complete water cooling set up...and you can get some specific water cooling cards...so in that i mean they have no gpu fan and just two pipes (inlet and outlet ) to connect to their pump
check out overclockers ( http://www.overclockers.co.uk/index.php ) since they have some of the best overclocking hardware and peripherals you can get
any problems let me know
good luck mate !