Broadband WiFi aerials

[#1]

No doubt this topic has been covered many times before but where ?
I have a Linksys WAG345G router and live in an old cottage with thick walls.  I have problems trying to connect wirelessly to the router all the time - I am lucky if I see 5.5 Mbps but very often it is down to 1 Mbps unless I am sitting right next to the router when I get 54 Mbps !
Linksys suggested all I needed to do was to upgrade the firmware on the router, which I have done, but it makes no difference.
So I went out and bought an aerial which scres into the router.  To be honest, I think the aerial might even make it worse.  Does anyone know if that is possible ?
If not, are there any tips for the placement of the aerial - vertical, horizontal, high, low ?  I have tried everything.  The router is upstairs, so therefore that should surely be an advantage.  I have even tried putting the aerial in the loft, because it has a 1.5 meter lead on it.
Any tips about aerials or improving signal strength would be most appreciated.

[#2]

I've had lots of trouble with a netgear - and only a solid brick wall. However, the PC's aeriel was behind about 2 feet of solid metal PC.
I bought one of those antenna on a 2m wire (cost £10 new on ebay). Magic! I went from unstable 1Mb to solid 54Mb. The aeriel was quoted as being 9Db gain (those little sticks they sell them with are only 2Db).
Make sure both aeriels have the same orientation (vertical), are at about the same height and are orientated "facing" each other.
All else being equal, I suppose high as possible should help.
Don't put a really directional aerial on the router - unless you have to. It will make life difficult for laptops etc.

[#3]

Thanks - I also bought my aerial on Ebay but my router has no external antenna - it is all internal but there is a connection for an external aerial which I am now using, so it is not a question of them facing one another in my case.

[#4]

homeplugs - sorted
I have some old 802.11 wifi kit from the days we did IAP - abit big for your needs but ive got an idea.