Ok here goes. I have a Compaq presario which has a dual core 2.63ghz processor with 4 gig DDR3 ram and a recently purchased MSI Geforce GTX 750ti. I checked the Mobo and an intel core quad is compatible with this board(socket775). Ordered the chip. Rubber gloves, thermal PASTE etc, was very careful when installing the new CPU. POWERED up and i just get 2 long beeps and no display. Put the Dual Core back in and everything works fine again. Is the chip dead or could it be a power problem and there is no BIOS update for my Mobo on the HP website. The GTX 750ti requires no extra juice from the PSU by the way. It's a pretty hefty GPU but low powered. This is bugging me to *censored*. I have the option of the CPU being replaced from the seller but is it a fried processor. As i said when i put the Dual Core back in it boots fine.
Please help people:-(How did you determine which MBoard is in that Compaq ? ?Checked on the board, model was right below the CPU socket.
H-IG41-uATX. REV 1.0 then was able to determine which CPU's were compatible with that board.No core-Quad listed here...

SEE Here...REALLY?? Rang my local PC specialist and gave him that specific Mobo number and he said get a core 2 quad q6600 socket 775. I've got a feeling an Intel Core 2 Quad is not a Quad Core chip. Either way it didn't work. The Engineer was adament on this particular chip.Also would this chip give me much more of a performance boost on FPS on GFX if it did work or would i be better of getting an Intel Core 2 Duo 3.16ghz that would almost certainly work.The Core 2 quad is in that link you provided but it just will not boot. I've tried removing RAM and varios other things and it just will not boot. It's so frustrating man.Return the CPU under warranty for replacement...Cheers Patio i knew there was something wrong with this chip. According to my research it just a BIOS update and a straight swap over and it should have booted no problem.Best of Luck with the replacement...let us know.It should be noted that HP gives a proper support list for this board...The Q6600 is not listed there.

Motherboard supports the following processor upgrades:

    Core 2 Quad Q8xx0/Q9xx0 (Yorkfield core) (95W)
    Core 2 Duo E8xx0 (Wolfdale core) (65W)
    Core 2 Duo E7xx0 (Wolfdale core) (65W)
    Pentium Dual Core E5xx0 (Wolfdale core) (65W)
    Celeron Dual Core E1xx0 (Conroe core) (65W)
    Celeron 4xx series (Conroe core) (35W)
Source: http://h10025.www1.hp.com/ewfrf/wc/document?cc=uk&lc=en&docname=c01903990#N116
Under the Processor Upgrade Information section.

The Q6600 is a Kentsfield architecture.

The Q8400 will be the appropriate replacement CPU to work with the board with similar specs to the Q6600.

If you want to go a bit higher, a Q9550 costs a little bit more (On Ebay anyway) but it is 2.83GHz instead of 2.66GHz.

In my opinion price for performance gain you are better off saving a bit more and buying a new Motherboard + CPU + Ram upgrade KIT. Unfortunately Compaq cases are usually heavily designed to fit the components that they supply. This means that things like the front panel connector and power supply connectors may not reach the new board properly meaning modifications have to be made and the power supply replaced to support the new motherboard.

Because you already have a dedicated graphics card to use you could purchase something like this:
http://www.newegg.com/Product/ComboBundleDetails.aspx?ItemList=Combo.1622704

I realise the outlay is 4-5x more, but in the long run will be much better performance wise.