A 1.8 northwood should overclock easily to 2.4-2.6ghz on default voltage, and would be a nice match for a 6600. I have a 6600 in my backup rig, and it works just fine, with a 3.06ghz northwood. Haven't tried HL2 on it yet, but I'm sure it will run fine at 1024x768. The 1.6 and 1.8ghz northwoods are some of the best overclockers. I have had 2 1.6's that ran at 2.4ghz on stock voltage, and I have a mobile 1.6 thats run at 2.7ghz with a little extra voltage. That gigabyte board you were looking at should be good enough to get your 1.8 to 2.4 without having to worry about voltage. Not a gurantee of course, but a really good chance.