Networking & wireless: Help on wireless connections!!!!!! =(

Help on wireless connections!!!!!! =(

Hi guys. I have horrible wireless connectivity in my house and have spent a lot on equipment to boost the signal, to no avail. Here's the rundown:

House:
-$800,000 with brick chimney, stone walls, no wired ethernet, no powerline capability
-Bad router placement in the office to one side of the house, impossible to move

Clients:
-Two computers with wired ethernet, one with Kaspersky 7 Antivirus, one with Avast free, both with XP Home SP2 with all updates
-Two wireless-g laptops, one with Vista with all updates, one with XP Home SP2 with all updates, both with Avast free
-My wireless-g desktop all the way at the OTHER end of the house!! *trouble machine, even with Zonet PCI card adapter, 5db high-gain antenna, 7ft. Hawkings extension cable, homemade Windsurfer (with XP Home SP2 with all updates, Avast free)

Equipment:
-SBC DSL SpeedStream modem
-WRT54Gv6 with dual 7db high-gain antennas with two 10ft. extension cables, factory firmware, (I'm scared of bricking it with DD-WRT, novice?)
-Belkin wireless-g range extender placed in the center of the house, 5 bar signal from router

Symptons with my computer at the other end of the house, (the laptops are fine because they are closer):
-Continuous dropouts if I connect to the router's network, 1 bar, 18 mbps
-Unable to obtain IP address if I connect to the wireless extender, 4 bars, 54 mbps, 100% requests time out if manually configured

What I'm looking at:
http://www.newegg.com/Product/Product.aspx?Item=N82E16833156226
http://www.newegg.com/Product/Product.aspx?Item=N82E16833164110