Need Advice - Wireless Routers?

I am having trouble setting up a wireless network in a concrete
building.

The company I'm working for has had a wireless network set-up,
complete with a di-link DI-624 Airplus Ext. G router, and 4 computers
with installed di-link DWL-G520 Airplus Wireless PCI cards. The network
worked fine - the furthest of the computers was 22 meters away from the
router.

Just last week, however, they moved from their small shop to a 3
story concrete building, adding a few computers, printers, and making
distances to a central point a maximum of 80 Meters. After testing
numerous spots, it seemed that the best spot for allowing all the
computers to connect now resulted in VERY LOW to NO reception. It seems
the computers are just out of range. I assume this is all the
concrete (as they use a lot here). Especially because the furthest
computer can reach the router, but the computer one floor directly
below has trouble staying connected.

With the company desiring wireless above any wired situation, I
wanted to know what anyone would recommend. My first thought was a
better router, i.e. MIMO technology, or some kind of booster. Would we
need new PCI cards too?

I'm open to any suggestions, and recommendation for router brands.

Thanks,
Brian