That white powder is likely to be dust or factory fluff, esp if you've never used the AGP slot (and you're not a drug lord of course)
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I'd rec getting a 2nd hand gfx card, pref a 4x & 1.5v card (same as GF4) so if it works you can be almost certain the GF4 will without actually risking it. Bear in mind though that your slot may not take anything other than old 3.3v 16MB cards depending on how old it is, so if a 1.5v cards doesn't work then try one of these older cards.
What CPU & mobo chipset are you running? Have you checked your case cooling and PSU will be up to the job? I'd rec getting an AthlonXP1900+, 512MB DDR, KT333 mobo, a new case and PSU. You should find that's under $300 (Pricewatch) and then a GF4TI4200 ($160) will be infinitely better than sticking a GF4TI4600 with an inadequate CPU (< 1 ghz). You'd have fewer probs, better future proofing and better performance in games and non-game stuff.
Just my penny's worth, peace
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