Yes, it all comes down to the lowest supported resolution of the monitor. 640 x 480 is the resolution it supports AKA VGA. A lot of modern monitors only support upwards of 800 x 600 AKA SVGA. My Samsung Syncmaster 15" only supports SVGA and XGA so I can't use my DC on it

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The best way is to just check with a laptop or anything portable that can output VGA resolution over a VGA cable. They might get a bit pissy with you if you take a DC or something that has to be plugged into the mains. I don't know how it is in your country but when I was purchasing my Viera they wouldn't let me plug my own equipment into their plug sockets because it wasn't PAT tested. They happy to demonstrate with similar equipment they had in store though.
With it being a relatively small purchase they may be a bit arsy about the extra work but at the end of the day a sale is a sale so you might luck out and get somebody who will do anything for you (the way it should be in my opinion, it's their job to sell after all).
Personally, if it HAS to be TFT then I'd try and get an old native SVGA monitor, chances are it will support VGA as well but it's worth checking all the same. I'd personally get a CRT as it will give you the best quality picture regardless of resolution. The only problem is the sheer size of the damn things, plus they eat electricity if used for long periods of time.