This is just my opinion as a developer, knowing the specs, current requirements and possible future requirements to play MWLL so take it with a grain of unbiased salt. The baseline specs I'd recommend for a PC is as follows. Please budget accordingly as this is just a baseline measurement for adequate performance and an explanation of how every part of your computer affects MWLL performance.
CPU should be 3.2+Ghz Quad-Core (Crysis isn't fantastic at Multithreading, but will still use two cores, and physics can be put on a third). Our game is extremely CPU intensive, so if you're going to sink a buck into any particular part, it should be the best CPU (preferably an i7 as its 4 cores are registered at 8 [with Hyperthreading], which gives you better overall performance as the extra cores can be put on system and background tasks). Our game should have at the very least 2 cores to itself, with 3 being optimal (push physics to third core) and let background tasks run on your remaining cores.
RAM should be 4GB minimum (6+ recommended) as with modern-day operating systems (Windows in particular) I run roughly 1.5GB at a fresh boot (Windows 7 pre-caches RAM to be used for commonly accessed programs, so your computer isn't actually using 1.5GB the same way WinXP would, as this increases overall performance at the cost of pre-cached memory). MWLL generally uses 1+GB of system memory, but to avoid "paging" (paging means accessing the hard disk to be used as temporary ram, which decreases overall system performance as your hard disk isn't as "fast" as ram). So, in summary, by default, with a fresh boot of Win7, no background programs running, and just MWLL, you're looking at almost 4GB ram optimally without any performance decrease.
GPU should have at minimum 1GB memory for optimal performance. You may skate by with 512mb, but you will notice stuttering at higher resolutions and texture settings simply because MWLL also uses quite a bit of video memory due to the amount of assets loaded on screen. MWLL is a different beast than Crysis as the maps are generally much larger, as are the assets themselves (larger assets require larger textures as they take up more actual world-space than Crysis assets). To retain visual fidelity, larger texture sizes are required per-asset, which increases the amount of video memory used. I'd recommend a card with 2GB ram personally to future-proof yourself for this game, and others.
Sound Card should have at least 128 channels, 64 minimum to avoid the dropping of sounds. Generally, sound cards don't matter much in current systems, but for some reason, due to the amount of things going on (multiple weapons firing from your mech and others around you, collisions, sound effects, so forth) our game is a bit more sound intensive simply due to the amount of things actually going on around you. This is just recommended, but not as highly recommended as the above parts. Regard your sound card last in the priority list.
My personal Recommended MWLL System Requirements for optimal performance:
CPU = 3.2Ghz (or higher) Quad Core i7
RAM = 4GB (or higher)
GPU = 1GB (or higher)
SC = 128 Channels
My personal Minimum MWLL System Requirements for sub-optimal performance:
CPU = 3.0Ghz (no less) Dual-Core
RAM = 4GB (no less)
GPU = 512MB (no less)
SC = 64 Channels
Obviously there are other things that can affect performance, such as hard drives. Getting a 10k RPM hard drive or SSD will decrease loading times of launching MWLL and loading maps and increase overall system performance dramatically. Faster RAM (DDR3 1333+) will also decrease loading times and speed up the launching of applications and so forth, but it really comes down to finding a nice median for your budget.
If you can't afford a system with the recommended system requirements, you can build a baseline PC that can run MWLL (and any other game) fairly well with $500-$700. Anything higher than that is just performance gravy.
I hope this helped.