Signal quality at a PC is poor and unstable. I want to correct this.

I have just moved my router from downstairs at the back of the house to upstairs at the front, leaving a PC downstairs at the back.

The PC used to get a good signal strength and speed when it was near the router. After the move, the signal strength fluctuates from good to low (and sometimes disappears altogether) and the transfer rate is rarely 54M, usually 18M or 11 M sometimes only 1M.

Which is the best way of dealing with this so i get a strong and consistent signal?

To the best of my knowledge I have the options of:

+ getting an extension lead for the antenna on the PC's wireless PCI card and raising it higher
+ getting a high gain antenna for the router (but what dB strength?)
+ installing a Wireless Access Point

But I don't know which will work if any (is there anything else that is better?)

I don't really want to install a WAP - the reason I installed wireless in the first place was because I can't really install ethernet cables through the house and WAP requires an ethernet connection to the router, right?

If it's important, my router is Linksys WRT 54G and the PCI card is D-Link GWL-D520+

TIA

Brian