Obviously price is your major factor. Meaning, if you could get a 4GB SD card for only $12, you most likely would want the bigger card, right? That's what I'm assuming with this response.
So, given that theory, I think what's most important here is for you to know that you do not need a "fast" SD card for this application. All SD cards read at the same speed -- the "fast" cards (advertised as 20X, 50X, etc.) only write at faster speeds, and the cost of such "fast" cards is associated with the speed. Once you copy TomTom to a slow (normal) SD card, 99.9% of the time all you will be doing is reading from it anyway. So don't waste money on a "fast" card for use with TomTom. If you're planning on using it for other things (digital still or video camera, etc.), then you may want to invest in something faster.
I'm emphasizing the slow cards because they are rather inexpensive now. I just purchased a couple of 1GB Kingston SD cards at Buy.com for only $12 each after rebate. Kingston rebates tend to be very fast and after about 1/2 dozen rebate purchases of Kingston products, I have yet to have problems with a single rebate. A 1GB card should be enough for TomTom Navigator 6, and should be able to store the entire North American map on it.
As for breaking up the single big map into smaller ones, I don't think you can do that with Nav 6. You were able to do that with Nav 5, but that strategy resulted in the disappointing limitation that you could not do cross continent navigation (like plan a trip from Florida to California). You were limited to navigating within regions of the USA (SE, NE, SW, NW, etc.). Although this may not be a problem for you if you're only navigating within your state, it can be a problem if you want to go from North Carolina to Virginia, because that's where the region division is. You can also download state-only maps from TomTom, but that will cost you extra money. I think your best bet is to buy an inexpensive 1GB card, store the entire North American map on it, and just go from there.
Here are some links to current sales:
Kingston Pro Elite 2GB SD Card $29 free shipping
OCZ 1GB SD Card $11.99 with $5 shipping
Kingston 1GB SD Card $16.99 with $5 shipping
To get the same deal I got at Buy.com, you'll have to watch for their rebates/sales almost daily because their rebates usually only cover a 3-5 day purchase period.