system specs
celeron 2.4 ghz
512 ram

if it is fitted with a 6600gt will it be suitable?

or should I get a 9800xt instead

which is better for this pc 6600gt or x1600gt

If I was you, I wouldn't go for a 6600GT or a 9800XT.  A celeron isn't alot right now.  It'll bottleneck it for sure.  I'd suggest getting another stick of 512 MB and get a new processor.  Most of the current modern CPUs by AMD or Intel won't bottleneck (Don't get a Celeron or Sempron if you don't want to get bottlenecked).

One thing you want to keep in mind is that the card you buy for that won't survive your next upgrade.  Your Celeron almost certainly takes AGP bus graphics cards, but new computers and motherboards use PCI Express.  Don't plan on getting a nice card now, to keep while you upgrade the rest of the computer later -- that won't work out.

It would probably be best to not try to turn the Celeron into a gaming computer and plan on replacing it entirely.  If that won't be possible any time soon, figure out what games you are going to play (keeping in mind the computer's specs) and get the least expensive graphics card that should work reasonably well.

thanks for ur recommendations but I just want to know if the 6600gt will be bottlenecked by this celeron?
What card is the best card I can get without it being bottlenecked? both ati and nvidiav, AGP slot

I don't think you can get much to pair with it in gaming.  You could try a Ti 4400 but that isn't a top-of-the-line card however.

well I would have thought at least a 9600 pro if not a 6600gt