I like to use anandtechs bench ratings when I'm picking out my processors. AnandTech - Bench - CPU
They don't have every single one on there but it can give you a pretty good idea.
LGA 775 is a pretty old slot these days, Intel has released 3 new ones since then. I don't know how much you're looking to spend...
If you're just looking for another 755 processor to drop into your system:
This dual core has a bench rating of 191 for $179 - Newegg.com - Intel Core 2 Duo E8400 Wolfdale 3.0GHz 6MB L2 Cache LGA 775 65W Dual-Core Processor BX80570E8400
The cheepest quad core has a rating of 171 and it's $182 - Newegg.com - Intel Core 2 Quad Q8400 Yorkfield 2.66GHz 4MB L2 Cache LGA 775 95W Quad-Core Processor BX80580Q8400
So more cores doesn't always equal more performance. Higher price doesn't always equal more performance. That's why I use the bench scores.
I have an article on processor selection for custom builds on my blog if you care to peruse: Computer Building Basics: Selecting a Processor
Site is brand new, not even a month old, so i'm still tweaking. Just trying to get some core content up and then i'll focus on making it user friendly and pretty looking soon.