I've been a bit late to post about my very successful 4670 installation (and kudos to MXM upgrade !)
In my case I had no modification to do whatsoever, I just had to slot in the card and voilà it worked
I didn't even had to use the thermal pads since I've discovered that the X1800 was originally mounted with thermal paste... So I've just used my trusty Arctic MX-2
As for W7, the 32b version is OK even with the new card. Even though it has 1GB instead of 256MB for the X1800, it doesn't seems to bother W7 memory mapping. Therefore, W7 still recognises 3.25GB out of 4GB installed. As 64b version wouldn't be able to get more (I've checked with a 64b Linux DVD) since it's a BIOS limitation, and somehow I doubt that FSC or Alienware or anyone else will work on a BIOS with no RAM restriction.
And yes the DVI plug works like a charm, I'm using a DVI/HDMI converter and enjoying HD movies on my LCD TV. It works so fine that I'm even considering ditching my plans to build and HTPC and use my Xi15456 instead !
Indeed, I've ditched the two 100GB HDD for one SSD: SSD perf are not good with the xi1546 (because the @#%&!$ VIA chipset is placed on the PCI bus...) but 4K perf and ridiculous access times are there so you still feel a huge difference. But more importantly for the HTPC side of things, with only one drive there's no more hellish HDD right next to the graphic card... which means that the fan rarely kicks itself on even during games ==> perfect for an HTPC: no fan noise, no HDD noise !!! And since I've replaced my DVD drive for a BD one, I've got blu-ray capability too
As for games, I'm playing Mass Effect 1 at 1280x720 with almost all settings to the max (I've just switched off the Film effet and the dynamic shadows). And PES 2010 plays at 1980x1080 with all settings to the max
So yes, you won't play Crisis at full settings with it, but if like me you game mainly on your desktop (mine has a HD5870) and use your laptop as a backup or as a guest PC for multiplayer games, then the Xi1546 with a HD4670 is a GREAT option. In short I'm a happy man
The laptop config:
Xi1546 T7200 CPU with an 945PM chipset (same as Bjornski1976)
BIOS: Alienware 2.07 bios (supposedly it has improvements from 1.19
2x2G RAM: Kingston Value RAM KVR667D2S5K2/4G at 533Mhz (PC5300)
ATI Mobility Radeon HD4670 1GB (same as Bjornski1976)
1 Intel Postville 80GB SSD
Sony/OptiArc 5500A CD/DVD/BD-ROM drive