It doesn't make any difference. Really. An Atom processor would do good enough for Office 2007.
This.
I installed Office 2010 (the college-student Pro version, with Access as well as the rest of the standard Office programs) on my 3-year-old HP Mini 311 (Atom single-core 1.67GHz proc, 3GB RAM, IIRC I installed a 320GB 7200rpm HDD in place of the 160GB 5400rpm it came with; OS is Win 7 Home Premium), a couple of years ago. It runs like a top. Actually, there's no particularly-noticeable difference between running Office on that, and running the same suite on my new Sager desktop-replacement laptop (Ivy Bridge i7-3610Q, 16GB RAM, 750GB 7200rpm HDD, Win 7 Home Premium - I can get Win 8 for $15, but while I might get it, it won't go on the new laptop). Office should run pretty well on just about any modern computer. I'd echo Chris' sentiments and aim for an Core i5-processor system, if you can get one without going out of your budget (and I'd think you could - at worst, an upper-end i3). Load up on RAM, up to 8GB like he said, the SSD might be overkill and will raise the system price compared to a mechanical HDD but will unquestionably be faster - it's on my own upgrade track for the Sager, sometime relatively soon, as the boot drive. I've an HP laptop that's hanging in remarkably well, for its age (~4 years), but HPs do come with a metric butt-ton of bloatware. Been a while since I've seen what Dells come with, but I recall them being pretty high-fat as well. Really, any cheap computer you get is likely to be infested with at least a moderate amount of bloatware, I think.
Me, I wouldn't be comfortable with less than about a 240GB SSD as primary unless I had a largish mechanical secondary-app/data drive to back it up; if it were the only drive in the system, a ~240GB SSD would be the smallest I'd consider (I'd WANT more...), and I don't think I'd go below ~120GB regardless, because stuff WILL go on the SSD. Pics, music, programs...
