The E6600 is a Core 2 Duo, not a Core 2 Quad D3D.

I'd definitely go for a Core 2 Quad in a new PC, since you can buy the Q6600 starting at about 260 (!) Euros already (at www.azerty.nl , www.alternate.de etc. ..). I would advice not to buy a Dell PC, but instead buy all your parts in online stores like Azerty or Alternate.de using their 'pc-builder' thing. It's not just way cheaper, but you'll get really good hardware and service too in online shops nowadays. The only downside I can currently think of is that you will lack some pre-installed software. You'd have to buy a Windows Vista and Office 2007 for example. I'd recommend the OEM systembuilders version which can also be ordered at shops like azerty.nl . Vista Ultimate costs only 170 Euros or so and depending on the normal shop you'd buy your things this would be exactly the Windows version you'd normally get anyways.

As for specific parts, I'd consider a 3D card like ATI's HD2900XT or perhaps even a Geforce 8800 GTX, but needless to say both are quite expensive. You could also go with the Dell PC and buy a next generation 3d card by the time Nvidia's 9800 series comes out or ATI's R700. Both are expected to come quite soon already, supporting DirectX 10.1 (the 8800 and HD2900 can't handle DirectX 10.1, but then again I don't expect games to use it this soon anyways).

Ultimately it comes down to your budget and whether or not you'd like your PC to be good for another year or good for another 4 years or so.

Cheers