Intermitent Wifi connection problem on Inspiron 6400

Hi I have had my Inspiron 6400 (Windows XP SP3) for over 18 months now, and have been using it continuously on Wifi with my Thompson ADSL router.  I also have a Vostro 1400 and a Vostro 1500 (Both Vista ultimate) in the same home.

In the last week or so my 6400 has more and more frequently started to drop it's WiFi connection.  When it drops, it usually auto reconnects in under a minute.  But lately it's getting worse, to the point where it's unusable.  Sometimes it connects for a decent period as long as I'm just browsing, but as soon as I try to do any form of sustained data transfer (eg Download, or open/save large files on my network drives), it seems to start off fine then dies after a little while.

When I do have a connection, the info shows a quality of 'Excellent' and speed of 54M.

The Vostro's are not having any problems at all, so I assume the router is not the issue.

I'm getting to the stage where I'm considering replacing the mini-card, but at a cost of £60 I'm wondering if Id be better off installing an Express port Wifi card.

If I decided on the Express card option, can I configure the machine to ignore the internal card and use the Express card instead?  I'm thinking this would be done by 'dissabling' the device driver for the internal card, and then use the Express card's driver (I'm assuming the card would be supplied with/require a dedicated driver).  Am I on the right track? 

Regards,

Glenn