SFF systems are not upgradeable this is not supported.

Why do you say this, it is upgradeable take a look please - https://www.youtube.com/watch?v=8H1eJtZDkqY he has the same pc as mine and he even upgraded the hard drive to 1TB and the ram i think to 8GB, he upgrades first, in a previous video, from Core 2 Duo E4400 to Core 2 Duo E6600 then he upgrades to Core 2 QUAD Q6700 and it clearly works, this is proof that it is upgradeable, he upgrades twice AND if you look closely he also has a graphics card so not only does he upgrade his cpu, he upgrades with the graphics card in the pc and it works, pleas tell me what you think, Ive noticed you answer a lot of questions and I need your help to answer mine please, OH and I forgot to mention that I updated my bios to latest version 2.6.6, Thank You.

All I want to know is if I can upgrade the cpu to Core 2 Quad Q6700 WITH the R5 240 (50w) graphics card in it, because I know that if the graphics card wasnt in it, it would work, but I need to know if it will work with the Core 2 Quad Q6700 (105w)  AND the R5 240 (50w), will it go over the 275w power limit? ANY help is appreciated, Thank You

Dell does not support ANY processor change.  The system he had uses the COPPER performance heatsink not the Aluminum one. There are lots of caveats so I still say what you want to do will have issues.

He is using SLACQ INTEL CORE 2 Quad Q6700.

He also DID NOT SAY which BIOS he was using.

Recommended update to last known bios BEFORE changing anything.

For a 745 the HEIDEN_2.6.6.exe is recommended.

https://downloads.dell.com/FOLDER00224415M/1/HEIDEN_2.6.6.exe 

Power supply is not upgrade able.

Official spec DOES NOT LIST QUADS

https://www.dell.com/downloads/global/products/optix/en/opti_745techspecs.pdf

The 755 lists quads the 745 does not.

The other issue is with RADEON R series cards.

THEY DO NOT SUPPORT DOS VESA VIDEO MODE 103 and therefore black screen don't work at all ever.

Sapphire and Powercolor will tell you they only support UEFI BIOS.

Earlier RADEON HD SERIES HD 2400  3400 4600 5450 6450 7750 work fine.  NONE of the R series work.  So asus EAH 6450 is fine.

Hey speedstep im currently using an R5 240 1GB Low Profile and it works perfectly fine never had bluescreen or black screen, ALSO, I check the gpu temps very freequently and it never goes over 135 degrees fahrenheit, when idle its 86 degrees, simple tasks its 90-100 degrees, playing games, 105-125 degrees, so the gpu is perfectly fine and I have a copper cpu heatsink I removed it and its copper under. Should I upgrade to Core 2 Duo E6700 instead, BECAUSE, it uses 65w maximum just like my current Core 2 Duo e6400, and since a Quad can work im pretty sure an E6700 will work please let me know what you think. Also HEIDEN_2.6.6.exe is the exact same one I used from dell.com, and worked sucessfully, at startup it says BIOS 2.6.6. Thank You for the reply.

"R5 240 1GB Low Profile "  and how do you get into F2 BIos?

Which Brand/Model card is this?

Sapphire and Powercolor and quite a few others just say we dont support non UEFI. Getting into Bios does not work because DOS VESA Video mode 103 is not supported.

Im sorry I wasnt specific, its an AMD Radeon R5 240 1GB Low Profile, I understand the UEFI and BOIS thing, because I decided I was going to buy a Sapphire Geforce 710 2GB Low Profile but I saw in the comments below that someone bought it and he said, "it didnt work on my computer because its a UEFI card and my pc is BOIS," so I was very careful when I was deciding what to buy, AMD Radeon R5 240 1GB Low Profile. I hope this answers your question, Thank You.

"Low Profile" is not a Brand Name Like ASUS or MSI or Sapphire or Powercolor or Gigabyte ETC.

Its also not a model number.  You also did not say how you got into F2  bios setup with the R series card ASUS for example is R7240-2GD3-L

There doesnt appear to EXIST an R5 240 but rather R5 230  as in R5230 SL-2GD3-L

https://www.asus.com/us/Graphics-Cards/R5230SL2GD3L/

sorry, wont say low profile next time.

speedstep when I inserted the card I just turned the computer on like normal WITHOUT having any R5 240 drivers installed at all, I was very surprised it showed picture without drivers BUT when the computer turned on it was 800X600, when I went into device manager under Display Adapter it said I think Standard VGA Adapter, then I installed drivers and it went up to I think 1980X1080 and said R5 200 Series instead of Standard VGA Adapter, I never needed to enter the F2 setup in BIOS the computer loaded normally, Starting Windows, Password prompt then Welcome, the only issue BEFORE installing drivers was that It wasnt using the R5 240 to run my games even though the DVI cable was plugged into the R5 240, after installing and updating drivers it worked perfectly fine, games running faster,  ALSO I overclocked it, downclocked it, changed the fan speed, nothing wrong, only problem with that is when I overclocked it from I believe 850mhz core clock to 1000mhz core clock, temp didnt go up much at all just 5-10 degrees fahrenheit, the display lagged for 2 seconds then the display froze completely, I force power off then power on, back to normal, I dont play using overclock I was just testing, I play games with regular core and memory clock settings.

What do you mean by,   There doesnt appear to EXIST an R5 240 but rather R5 230  as in R5230 SL-2GD3-L

was there something I had to do in the F2 BIOS setup?