Is it worth using about 20 old dell workstations to mine at once? They have ancient hardware from about 2003-2005. Would it be better to use than one computer from modern time?
I wouldn't use them to mine.
Since GPUs back then couldn't do arbitrary calculations, the only thing you can use are the CPUs. CPUs are relatively better at mining scrypt (e.g. Litecoin) than SHA256 (e.g. Bitcoin), so let's go with that. Based on the oldest CPUs listed (which aren't quite old enough) on Litecoin's wiki, I'm going to guess that each computer can do 5 kH/s, which is probably being too generous. So that's a total of 100 kH/s. Plugging that into a calculator, even with assuming (another probably-too-generous assumption) 500 W total are used (that wouldn't otherwise be used), you're losing money if you have to pay for the electricity and/or cooling. (making maybe $0.14 per day, pre-expenses)
For comparison, a modern GPU can mine around 1400 kH/s for ~500 W ($2.01/day pre-expenses). Or a dedicated Bitcoin miner could actually turn a profit ($4.84/day pre-epenses) in a reasonable amount of time (of course, you're basically betting on/investing in the future of Bitcoin by buying something like that, which involves a good deal of risk/uncertainty).
From a power standpoint it would be much more expensive than mining with a normal machine. Bitcoin CPU mining has been phased out by GPUs and GPUs have been phased out by ASICs.
There are some coins which are more effective by CPU or are not yet implemented in GPU. I would say this is your best possible chance at netting a return even on just the power consumption of your rigs. There is a well-maintained list available at http://cpucoinlist.com/ make sure to google the algorithm + GPU (e.g. "X11 GPU") to make sure there is no GPU mining.