hi mate
overclocking has become alot easier with the introduction of motherboards like the asus P5Q SE2 ... with unlocked multipliers and adjustable cpu core voltage its never been safer to overclock a cpu... however there are still risks involved when increasing the frequency on a cpu however with the right cooling( both cpu and case cooling ) you should be fine
first of all you need to update the bios on your motheboard... this isnt essential however some updates add additional features and more stability... however if your happy with the current bios tweak settings ect then leave the bios update
cooling as i have allready mentioned is paramount to any succesful overclock... however since your overclock is only small you should be fine with the stock cooling.... but make sure you have adequate case cooling such as a 120mm fans at the front and rear set up in a push pull configuration...( air in at the front .. then exhuasted at the rear ) .... this will ensure a steady supply of air through the case and will also assist the expelling of warm air thats moved from the cpu
next theres the software... and even if you are overclocking through the bios you still need these programs/utilitys.... these are
1.cpu-z ( http://www.cpuid.com/softwares/cpu-z.html )
2.prime 95 ( http://files.extremeoverclocking.com/file.php?f=103 )
cpu-z is used for temperture monitoring and also shows the fsb and multiplier..... this is used by enthusiasts to keep a tab on tempertures and core frequencys,,,
prime95 is stability utility... it stresses your pc to 100% cpu usage over specific periods of time... most enthusiasts will leave prime 95 running overnight to ensure the system is 100% stable,,, if this fails then you need to lower the cpu frequency and run again
to overclock your cpu you can use software or the bios... asus do a overclocking utility called "smart doctor" this should be on the driver/software disk that came with your motherboard however you can download it from http://www.techspot.com/downloads/3505-asus-smartdoctor.html
using the software is much easier as its been designed with safety in mind and wont let you do any massive increases... and for novices its probally your best option...
however overclocking via the bios requires first accessing the bios... you can do this by pressing delete during boot.... then look for the tweaking options.... these may be called "performance tools" or "cpu overclocking " ..... its different on each motherboard however when you see it you will know .... you will see options for fsb,cpu multiplier and frequency
small overclocks can be done by simply changing the fsb(front side bus ) and to work out the cpu frequency you multiply the fsb with the multiplier.... so 13.5 x 200mhz = 2.7ghz.... however by changing the fsb to 210mhz this will change the core frequency to 2.83mhz...... push this even further to 225mhz fsb will give you a 3.03ghz cpu frequency ....
the best and safest method is to increase by 5mhz increments and then check the system will boot.... if it runs ok then add another 5mhz.... however set a target which in your case is 2.6ghz ... theres no use in just continuing until your pc blue screens or wont boot....
then theres the multiplier,,, in overclocking you can use this to acheive results... however some motherboards lock their multipliers (apparently to stop support calls ) however if the multiplier is usable then this can be used in conjunction with the fsb to acheive the target speed...
the method here is to lower the multiplier but higher the fsb to.... however the multiplier can also be raised and then the fsb wont need to be increased as much... however at the end of the day its all about stability.... and thats why you need prime95
once you have reached your target speed remember to save and exit set up( on the main bios screen ) ... this will save the settings to the cmos (permanent bios memory ). .. then you pc will restart as the changes are made
i dont think there will be any problems with the GTS250... these cards were designed to run on systems much like yours... its only when running GTS250,s on older athlon64/pentium 4 based systems were people report bottlenecking ...... you will be fine
finally overclocking requires a decent psu... however your BFG 650w psu is ideal... a good brand and extremely stable... so this is great
i hope this has helped,any problems let me know
good luck mate !