I've got an X250 (2C/4T) 16GB gutless wonder as my Qubes laptop, and it's fine. Qubes has memory balancing/clawback from VMs, so you can in practice have more AppVMs open than you'd expect - they'll be using less RAM, but it works.
If you're on a short-RAM laptop, though, definitely reduce Dom0's memory allocation. It defaults to 4GB, and you're perfectly fine with 2GB or perhaps even 1GB - there's not much going on in it, and that RAM is better used for AppVMs.
If you're short on cores, you might also look at the "sched-gran=core" flag to Xen. This allows hyperthreading, but ensures that hyperthreads are only ever scheduled in the same VM (and as the threat model assumes "anything in a VM can read anything else in a VM," a hyperthread-based leak doesn't gain an attacker any access you wouldn't otherwise have). The performance gains on a laptop can be noticeable.
Don't expect great battery life, though. Xen's power management is "present and accounted for," at best. There's also an incantation to disable turbo that helps a lot when mobile.