Since you haven't gotten a reply; I think what the previous poster is avoiding telling you is that your second link doesn't have the UFL to RP-SMA adapters included so you can't attach them to the WRT320n. Not only does the first link include those, but it looks like it would a easy for anyone who can handle a screwdriver to install them. That said, I haven't done this mod to my WRT320n yet so I can't vouch for its performance, but given my experience with this wireless router's internal antennas I don't see how it could hurt. I hope if I'm overlooking something, someone will step forward instead of saying "if you don't already know I'm not going to tell you"... it's not helpful.
If you can find the UFL to RP-SMA adapters as in that kit, then you can use the 9dbi antennas or even a better YAGI or home-grown antenna. I think this would work... but do your research before taking my word for it: http://www.sparkfun.com/products/662