Hi There,
There is a short thread about just this in the MAC forum, have a quick look. In summary, macbooks in the US are excellent value. But you run the risk of being stopped at customs. If you're buying 1, the best thing to do is chuck the packaging and bring it back in your hand luggage. You could pack the packaging in your checked baggage, but you run the risk of that being checked. If you post it to yourself in the UK, customs will almost certainly stop it. Then say hello to Mr VAT man. Be very careful with the likes of TNT, they pay all charges upfront and then charge you. Gives you little scope to appeal them.
Personally I'd just carry them both back in my hand luggage. The chances of getting stopped are small, but you have to be except that it's a risk and that there are consequences.
Anyway have fun in NY.
I've heard of people taking a copy of Office with them and some pictures, music and work on a memory stick. Install Office and copy your work onto it in the hope Mr Customs won't stiff you duty + tax. There was nobody about in Customs when we came back through Manchester airport but I'm sure somebody was watching somewhere...
Leatherman multi-tools are good value over there. I got a charge TTi for ~£55. They are £110+ from decent retailers in the UK. The NikeTown shop is worth a visit, some decent savings to be had.
Anyway...
Empire State - open till 1am, so go on a clear night to avoid the queues and get great views.
Top Of the Rock - we went first thing one Sunday morning - no queues and no crowds.
Oyster Bar @ Grand Central - great place to eat if you like fish. If you don't love fish, don't go as out of A3 sized menu, 3 items weren't fish.
Heartlands Brewery - three bars in Manhanttan. One is near Times Square, another near the Rockfeller. Top beer

Staten Island Ferry - Free and great way to see the Statue of Liberty.
Central Park - Check out the size of it from the Top of the Rock and if it's still open, go ice skating.
We saw the NY Giants play (and lose - rubbish!). Get the shuttle bus from the PA Bus Terminal - takes about 20/30 minutes going but over an hour back with queueing to get through the Tunnel.
Red Roof Inn was a good place to stay. Two blocks from the ESB, clean and comfortable. Nothing swish but then you're in NYC - the only time you should be in the hotel is when you're asleep!