A repeater is not a good option if you have a very poor signal up there.
Your best bet would be to place an AP (access point) upstairs and connect it with a CAT5e cable. If a cable is not feasible you could use a pair of powerline adapters to get the signal up to the AP. Any inexpensive G wireless router could be configured as an AP.
Depending on your house, an N wireless signal from a high quality router could also potentially solve your problem, although you would need N capable adapters or devices on the other end.