windows 7 and 8 is 64 bit so the memory issue is just a scare. if all you are doing is video and graphics macs are best but remember most of the world and other devices use windows and they are v v different.
If its for general use + video editing then its a PC or laptop.
Next memory is v v cheap. check for your self and look at how much an extra 2 or 4gb will cost you so dont be put off by buying a machine that has 4 or 6 Gb of memory.
something like this is fine:
if you dont have windows HP Pavilion G6-1344sa 15.6" Laptop - Intel i5-2450M 6GB 750GB HDD HDMI DVDRW W7 | eBay
as for i7 the extra money is not worth the extra power in percentage terms so unless you are doing loads and load of it then a few seconds wait will save a few hundred pounds.
Oh and before someone goes on about hard drive size you would get around 250-300 films on the disks of the above and your other applications and I dont think you will be making that many 1.5 hour films! yes you will have other stuff as well but please come back to me when you are even 50% full as hard disks are getting cheaper every day and it will take you a good time to fill the ones in the links above up!
Ps I only have 20 years IT industry working for resellers infrastructure i.e. pc's , servers etc.