welcome to myadsl!
extenders will half you bandwidth available, so if possible, steer clear of them.
a new antenna could work, but should be used as a last resort as there are other options.
sometimes you can improve the signal by moving the router around. wifi needs line-of-sight, but the signal can bounce off and go through walls.
also make sure there is no interference from other equipment nearby. things like microwaves and fridges can interfere.
you could also try a different channel. to check which wifi channels are in use, use
Network Stumbler. it will show you all wifi signals your PC can see. make sure you use a different channel to the ones currently in use.
you might get more help on a dedicated wifi forum. there should be a WUG (wireless user group) in your area (Cape Town, Pietermaritzburg, Pretoria, Durban and Johannesburg). you might even find someone to come to your house and help you out!
SA WUG (fairly new site)
Cape Town WUG (been around for about a year)
Johannesburg WUG (oldest WUG I know of)