I want to get a laptop that uses ATI/AMD 5870M Crossfire (laptop) does that make it so I need a better processor? Would a i7-720 cut it first generation i7 cores for gaming in like BF3 COD:MW3? What about Crysis? The laptop I want uses the first gen Alienware M17x R2 (around 1200) for the set-up I buy USED. The M18x just came out as used, pretty pricey like 1500-1600 for the cheapest ones (I think I only saw one at the price or 1700 with SLI/Crossfire). Will the i7-720 suffice if I want like Max settings?
If gaming is the primary purpose then a desktop would better suit your needs. If you want a laptop to play games and take to class or business trips then you just need to realize the limitations before taking the plung.
1. Most laptops do not allow you to upgrade the video card. If the laptop does have that capability, then finding an appropriate video card (MXM format) will be a bit difficult. Your best bet would be ebay.
2. Many laptops have CPU soldered into the motherboard. Even if the CPU is not soldered in installing a faster CPU is generally not advisable since you cannot upgrade the laptop's heatsink.
3. Laptop video cards are less powerful than their desktop counterparts. This is because of power draw and heat. For example, the Radeon HD 5870M is about as powerful as the desktop Radeon HD 5770.
4. Due to the small size of a notebook/laptop, they generally run a bit hot when playing games. For example, while playing Crysis 1 on my Lenovo IdeaPad Y470, the Core i5-2410m heats up to 93C, the nVidia GT 550M is a more comfortable 72C.