I posted this in a different thread this morning, but it looks like it may help you out / apply to you also.
Here are some things to consider:
GPU: The x600 isn't going to take you where you need to be, especially with Vista, granted it does support direct X 9 (vista minimum), but is so wimpy, it doesn't even make it on my mobile GPU comparison page. Consider upgrading to a more powerful card x1600 / 6800 variant is probably the best you are going to do in that pricerange. The good news is, one of those cards with 256 MB of RAM will last you a very long time.
CPU: You bring up 2 very good points... 64 bit, it's a must; and to dual core, or not to dual core... that is the question. The Yonah Core-Duo chip is a 32 bit dual core processor. The Turion, a 64 bit single core processor. The benefit of the Intel chip is a reported (by Intel) complete drop in upgradability to the Merom chip CPU which will be a 64 bit dual core and 20-40% (as reported by Intel) faster than Yonah chips. Intel is a viable alternative now that those notebooks can be upgraded in the future.
The AMD Turion X2, which is AMD's new dual core notebook chip based on the X2 desktop chip should be coming out this within a month.
ExpressCard Slot: Ok, when PCM/CIA came out it was going to change the world!!! I still have no use for mine, and I would like to hear from anyone who does use it. The one thing that it may actually be useful for is the SoundBlaster card to keep your external speakings hooked up and a simple push button to disconnect and reconnect them. Now ExpressCard is the new version of the PCM/CIA 2.1 card. Now it's really going to get good, as the makers and hypers of this technology keep saying, but I have yet to see one cool thing (like an ExpressCard universal docking station) this slot supports. The potential is there, and to have a laptop last 5 years, you should also consider this doo-hickey.
Hard Drive: You may want to consider the potential scarcity (and price increase that accompanies scarcity) of EIDE or PATA drive in the next 5 years and go with SATA drive.
Now, I would love to help you out, but the only 17"+ models I have are out of your pricerange as they are KILLER gaming rigs with dual 7800 GTX and 7900 GTX video cards. I do have a couple 15.4" models that would work, but that of course is entirely up to you.
Depends on what you're using it for.
General Usage: Nearly identical, except for the 32/64 bit right now.
Multi-Tasking: Right now about the same.
SLi (dual video cards): AMD is the only option available. Multimedia & Media rendering: Intel Core Duo
Future Upgradability: Intel Core-Duo (Yonah platform since it can upgrade to Merom and Turion's can not upgrade to Turion X2's (different slot))
64 bit: Right now AMD, but as stated above Intel's new chip can supposedly "drop right in"
Value: They are about the same.
I wouldn't, because I can't even wait 15 seconds for a web page to load.
Even when the new intel & AMD chips come out they are going to be hard to get and expensive (judging by past experience) for at least another month.
Why not get a core-duo system and upgrade it next year when the 64 bit Intel chip is dirt cheap? Look on ebay for Pentium M's... they can't give them away. Next summer it will look just like that for what we're waiting for right now chips.