It might be a combination of the wear on the chain and also the steel chainrings. Did your old crankset use aluminum rings? They tend to run quieter in my experience.
Getting a new 8sp chain doesn't have to cost too much, an inexpensive Sachs might set you back about $15. However, that would give you a new chain and used rear cogs, so things might get worse unless you replaced the cogs too. Having fewer teeth, cogs are more sensitive to mismatch in wear.
For an objective method of measuring chain stretch, get the chain under tension and measure a 24-link section (12 inches), from the edge of a rivet to the edge of the corresponding rivet 12 inches down the chain. I think the general idea is that you replace if it's more than 1/8" stretched, although I use a chain-stretch gauge.
The problem with a stretched chain on new gears is that only the first few chain rollers bear load, instead of it being distributed all the way around the cog or chainring.
Update: from Sheldon Brown's page on chain maintenance, some better instructions:
Measuring Chain Wear
The standard way to measure chain wear is with a ruler or steel tape measure. This can be done without removing the chain from the bicycle. The normal technique is to measure a one-foot length, placing an inch mark of the ruler exactly in the middle of one rivet, then looking at the corresponding rivet 12 complete links away. On a new, unworn chain, this rivet will also line up exactly with an inch mark. With a worn chain, the rivet will be past the inch mark.
This gives a direct measurement of the wear to the chain, and an indirect measurement of the wear to the sprockets:
If the rivet is less than 1/16" past the mark, all is well.
If the rivet is 1/16" past the mark, you should replace the chain, but the sprockets are probably undamaged.
If the rivet is 1/8" past the mark, you have left it too long, and the sprockets (at least the favorite ones) will be too badly worn. If you replace a chain at the 1/8" point, without replacing the sprockets, it may run OK and not skip, but the worn sprockets will cause the new chain to wear much faster than it should, until it catches up with the wear state of the sprockets.
If the rivet is past the 1/8" mark, a new chain will almost certainly skip on the worn sprockets, especially the smaller ones.
Last edited by mechBgon; 07-22-02 at 11:30 PM.