Your Quad-core intel should be more than enough. I use a quad 2.66 (2006 MacPro) at home and at work....the one at work has 4 gigs of ram and the one at home has 5. A few things will make this computer a video power-house.
1. If you have 3 open internal hard drive slots, fill them with 3 big hard drives (some 7200 rpm, 1 terabyte hard drives would do) and RAID them. This will create a large, editable, volume. Have your media digitized here, and you can edit from here...or at least set all your render files, audio renders, etc. to this volume. These you can find for $100 or less a piece.
2. The editing software. This is this thread's missing peace right now. If your post house is using Avid, you'll want a copy of Media Composer. If they use Final Cut Studio, you'll want a copy as well....Same with whatever software they use (the only other NLE i could imagine is Premiere). This is where you could get a break. Do you have kids in school or know any teachers? Use a student discount which brings the price of this software WAY DOWN. If you morally object to using educationally discounted software then this is where you might ask your post house to help out....but since you are going above and beyond by doing a rough cut I think the world will cut you some slack. You aren't personally getting paid for editing...so that might make it OK in your mind, also you are helping in post production so that might motivate your post house to help you out. This software costs:
Final Cut Studio:
From apple $1,200 -
http://store.apple.com/us/product/MA886Z/A?fnode=MTY1NDExNQ&mco=MjE1MTQ4NQ
Student version $700 -
http://store.apple.com/us-hed/product/MA886Z/A?fnode=MTY1NDExNQ&mco=MjE1MTQ4NQ
Avid Media Composer:
From Avid $2,500 (discs) or $2,300 (software only) -
http://store.avid.com/us/index.cfm?
Student version $300 -
http://www.studica.com/Avid/#Media Composer
Adobe Premiere:
From Adobe $800 -
https://store1.adobe.com/cfusion/st...ro&loc=en_us&store=OLS-US&categoryOid=1883562
Student version $350 -
http://www.studica.com/products/product_detail.cfm?productid=58362
3. External drives. These should really come from your post house or the post house should be billed for them. Look for drives that have 2 or more hard drives RAIDed, Firewire 800, and at least 1 terabyte. If you digitize to the local volume (from the first category), you can "media manage" to the external drive and send it off to your editor. Or if you want to save yourself some time you could just digitize and rough cut off the external drive and then send it off. But doing a media manage ensures every piece of media (including music, pics, graphics, etc.) you use gets to the editor.
4. A capture card. This is optional if you stay in the world of HDV (or DVCPro HD) as you can capture through firewire. But if you get any HDCam media you might need a capture card. This also depends on software...a Kona card or Blackmagic card would work for Final Cut Pro and Premiere, and a mojo would work for AVID. Google around if you want to pursue this.
I really believe that the machine you have is more than adequate, but with little investment you would be REALLY ready. The only reason i could imagine your current system would be running slowly is because of other programs running, slow hard drives (either internally or externally), or there is something else wrong with your system (least likely). Just make sure ALL volumes have at least 10% free and you should be good.