Vista is fine to use on a new computer assuming it has 2 gigs of RAM. Unless it is an entry level notebook with 1 gig ram or a netbook, you will be fine with it. Netbooks tend to come with Linux or XP installed though, so you would never have that trouble.
You can upgrade the RAM on a Mac, but it will add to the cost which you are mentioning is prohibitive. My opinion on Macs is they are a good value for what they provide. The problem is if you are looking for something that doesn't need to play game or do any sort of intensive editting, they are probably providing you with stuff you don't need. If you are looking at a high end PC vs Mac then there is not alot of difference aside from the OS.
For travelling you need to consider other factors as well other than performance.
How heavy/large is it? 2lb netbook compared to a 17 inch gaming monster. Need to know how much you are willing to carry.
Travelling can lead to theft/loss of valuables. The cheapest laptop you can get that does what you need it to is reducing the amount of loss you suffer if that happens.
Anyway. If it was me, I would buy
a netbook or
an entry level Dell.