which Intel 2 wire N adapter?

The 5100 or ????

My niece has an older machine with a pair of antenna wires (and an open mini PCIe slot), we don't want to take the machine apart to string a third. She is hoping to get another 6 months out of the machine before she splurges on a brand new box for grad school and is very shy about taking the thing apart any more than necessary. But she really wants wireless on the current box.

 

You don't have to use two wire adapter.
If you prefer Intel skip 5100 which is really bad and go for 5300 (it has three antenna connectors but will work fine with two)
Intel 5100 causes a lot of problems for some reason so it's better to go for 5300 with just two antennas rather that 5100 with two antennas.
Here is the test of "n" adapters by smallnetbuilder that also recommends 5300.
You can also choose pretty much any card by Atheros as long as it comes from trusted source but the same goes for Intel.

 

^ They should add my Intel 6300 Ultimate-N in that list. I would love to see its full potential

The third antenna is to take advantage of MIMO technology.

It will run N speeds fine on 2 antennas. Third antenna is optional for a better wireless-N connection.

 

is the 5100 really bad or just 'not as good' as a 5300.

Teresa has read and heard (in the lab, science geeks) that the 5100 is better than anything Atheros has tossed out there.

Not that we're spending a lot of scratch on this. A 5100 is around $15- on ebay from a US_based salvage dealer. But the less this machine is 'disturbed' the more likely it is to last the summer.

And yah, I know that I'm the one always chirping in on spend a few $$ on a new machine with warranty, but what are you going to do with a Kollege kid?

 

Intel 5300 is probably the only good Wi-Fi card Intel ever launched (not counting the 6xxx series as I don't know how good or bad is this thing yet).
Older cards like 5100 caused a lot of trouble, 4965AGN was notorious for not working with hidden SSIDs (where others have worked) and older cards like 2200 were complete junk and had all the trouble in the world.

I will not comment on i5100 being superior to all Atheros cards- I've tried to write that part of the post three times and it always gets me banned.

Let’s put it this way- since a lot of people have trouble with 5100 and most people and reviewers are happy with 5300- why take the risk?