Upgrading E5300 to another LGA775 Processor. Help!

[Meight]

Hello all! This site helped in me making a final decision a long while back on what GPU to get, so, I figure why not try asking here again!

I'm currently in the process of upgrading my PC.
Current specs are: 19in monitor (1440x900), 5670, Dual-Core E5300, 300WPSU, 6GB RAM, Vista (if taking OS into account is important)
Upgrading TO A: 23in 1920x1080 ASUS, 6850, AND I AM KEEPING MY CURRENT MICRO ATX MOBO IN ORDER TO SAVE SOME MONEY (but buying a different case to house it, of course, instead of fitting these new parts into this prebuilt PC). This is where my question comes in!

My mobo (info taken from the HP website) is a:
Manufacturer's motherboard: Asus IPIBL-LB
HP/Compaq name: Benicia-GL8E
Website link here if interested: CLICK!

It says I can upgrade my processor to one of these:
Intel Core 2 Quad (Y) Q9xxxx
Intel Core 2 Duo (W) E8xxx
Intel Core 2 Quad (K) up to Q6600
Core 2 Duo E6x00 (C) up to E6700
Core 2 Duo E4x00 (C) up to E4400

I figure Quad Core is better. So, I ask, what shall I do? Grab a quad core since, most likely, my current E5300 will be a bottleneck to my 6850? I was thinking Q9550 since the benchmarks show it to be good, though they are VERY expensive on the sites I checked out ($300+ on Amazon, Ebay,), so I found the next best quad core, a -yes- USED, Q6600 for ~$100 on eBay.
Due to my budget, I really cannot afford to buy a new motherboard and new "2500k, i7, Sandy Bridge" (or however it goes) CPU.

Thank you very much in advance and I hope you can be of help!

-Danny

P.S. Hope I can sneak this in real quick, but: what PSU should I grab to power this? Should 550W suffice or should I bump it up? I won't need anything like 1000 or 900w, I'm pretty sure.

 

[GI_JONES]

q6600 would be a decent upgrade for you. A 550- 600 watt psu is plenty. Get a good one though, like Corsair, OCZ,Seasonic, Antec or PC Power & Cooling

 

[sk1939]

550/650 would be more than sufficient I would say.

 

[Snowsniper2]

The Q6600 would run nicely And a 500-600 watt psu would do the job perfectly.
I have a core i5 2500 and a core 2 quad Q8400 and the core 2 quad runs only slightly slower then the core i5 so if you were to upgrade to the core 2 quad you wouldnt be that far behind n todays hardware

 

[Meight]

Hi everyone
Thank you for your replies. Good to hear the Q6600 would work well. Of course, if I can snag a Q9550 for a price that suits me well, I'll get that instead!

I'd love to hear what others have to think and what recommendations and input they could give me here.

 

[Meight]

Hope you all don't mind this bump. Would like to get more input on this, thanks