I was also considering the ZD8000 vs the Dell (9300). Basically, I would consider the following points:
There are numerous good and bad comments on the 9300 screens on this forum. I haven't seen truelife in person, but if the xps2 has the same screens as the test units sent out, it should be fine.
Also, HP offers WXGA+ and WSXGA brightview screens, so if those would suit you better, that's another factor.
HP weighs a few pounds more
power consumption, heat, fan noise:
I think the HP is worse in all these categories, whereas the dell is more efficient here.
HP with a 6xx (64 bit, better power management) Pentium 4 is more comparable to a desktop, whereas the Dell's pentium M is more than adequate for most tasks but is still not the best in others such as video compression or when brute force is wanted. But I think most people won't feel the difference in everyday use.
The xps2 with the 6800 ultra is definitely the way to go if you are a hardcore gamer and plan to use this computer for FPS games on high settings into the future.
Also, have you considered the 9300? I think the 6800 non-ultra is still better than the x600 (correct me if I'm wrong), and the price is at or cheaper than the comparably equipped zd8000.
I was very close to buying a zd8000, but the 35% coupon dell had sealed the deal for me.