For the rest of people, this thread started here
http://www.tomshardware.com/answers/id-1697208/cpu.html and I have proven that Q6600 will work here, also OP has copper heatsink already, so next is GPU.
Just in case for lazy people this is my proof of Q6600 in Optiplex 745
Where WF810 per http://www.google.com/search?hl=en&source=hp&q=WF810&gbv=2&oq=WF810&gs_l=heirloom-hp.13..0l3j0i30j0i10i30j0i30l3j0i5i30l2.1590.1590.0.3147.1.1.0.0.0.0.153.153.0j1.1.0...0.0...1ac.2.15.heirloom-hp.IZYucBoKReI
is Optiplex 745 motherboard.
And statement that Q6700 is working as well.
THIS POST APPLICABLE TO MT (MINI TOWER) FORM FACTOR ONLY.
Unfortunately no card that is more than 7.5 inches long or 2 slots wide will PHYSICALLY FIT inside the case - it was found that only single bracket video card can fit due to BTX MOBO, where card internals facing upward towards PC internals, not ATX, where dual card design is blocking another PCI slot by facing downward.
If you prefer the card with beefier fan, then the length of the card must not exceed 7.5 inches, otherwise only "skinny" single bracket, single slot cards can physically fit.
Pictures below will prove my point.
View shows no room for dual bracket
Single bracket only, or you have to cut dual bracket in half
Here you can clearly see only single place for the bracket
Visual explanation
7.5 inches explained, also shown "obstacles"
This is the picture of actual "skinny" single slot video card (not sure which model)- perfect, "glove" fit.
So, I found 2 GDDR5 7750 cards and one DDR3 4GB (GDDR5 is better than GDDR3, even if GDD3 offers 2 GB RAM or more), which confirmed in reviews to fit Optiplex gx620, 745 and 755 cases (the difference between those models are different processors).
Skinny, flimsier in terms of construction, completely single slot XFX 7750
http://www.amazon.com/XFX-DisplayPort-PCI-Express-FX775AZNP4-FX-775A-ZNP4/dp/B007Z3T5JC
A bit beefier cooler, not as skinny, more durable in construction PowerColor AX7750
http://www.amazon.com/PowerColor-AX7750-1GBD5-DH-Video-Graphics/dp/B00775OC28/ref=sr_1_1?ie=UTF8&qid=1362783129&sr=8-1&keywords=AX7750+1GBD5-DH
If you need Video card for working with high resolutions,
BUT NOT FOR GAMING, PowerColor AX7750 4GBK3-H Radeon HD 7750 4GB is
DDR3 card with huge 4GB memory, but is is not as fast as DDR5 and loosing in games
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131486&Tpk=7750&IsVirtualParent=1
Here is review of this card http://news.softpedia.com/news/PowerColor-Launches-Single-Slot-Radeon-HD-7750-4-GB-Video-Card-292712.shtml so this one is not for gamers!
I cannot say which one of those 2 DDR5 is better, both are good and got less bad reviews related to structural build.
Next will follow maximum possible upgrade, 7750 is weaker...
So far I found only 2 7770 video cards which will fit inside - PowerColor AX7770
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131477
HIS H777FN1G Radeon HD 7770 very tight fit, almost touching heatsink
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161433
Following card is already out of production, so if you can find it on Craig list or eBay, it is only way, also I seen it on Amazon for a high price
http://www.amazon.com/Sparkle-SX650TI1024KD-SPARKLESX650TI1024KD--Grafikkarten/dp/B009ZC3MHM/ref=sr_1_1?ie=UTF8&qid=1371676541&sr=8-1&keywords=SX650TI1024KD
From NVIDIA I see SPARKLE 700025 (SX650TI1024KD) GeForce GTX 650 Ti
http://www.newegg.com/Product/Product.aspx?Item=N82E16814187200
7750 cards do not require power supply upgrade, 650 TI and 7770 are.
Officially 7770 requires 500 watt, which is huge overstatement http://www.hwcompare.com/11880/radeon-hd-7750-vs-radeon-hd-7770/ only 80 watt in reality, so 450 watt PSU you have should be enough.
anything else I can you with?
P.S. Products provided here are for USA retailers, Europe and Asia has a few more choices with Club 3D.