Also, given today's hardware, a 500w PSU is borderline average to totally inadequate, so given its age, and the danger of it taking out your other hardware if/when it dies, it would be wise to invest in a new, higher rated one sooner rather than later. It would also be wise to get something more powerful than you actually need... to allow for depreciation over time and accomodate future expansions/upgrades.
I have a Silverstone 1200w that is more than adequate in my current system, but it will only be a bit above average when it goes into my new build sometime in late Feb early March. A 1500w would be better, and I will likely invest in one when I've recovered from the cost of this new PC further down the track. However, for the time being, I will have to make do with the 1200w in order to purchase the remaining parts to complete the build.
Aargh. Probably regret jumping in but...
<rant mode begin>
Unless you're running multiple high-end graphics cards or a 5-disk RAID array or something equally unlikely in a consumer desktop machine, 1200 Watts is way more power than what you need, and 1500 is just insane. It just makes your power consumption highly inefficient because you'll be drawing much less than what the unit is specced for; most are only efficient if well paired with their intended power draw.
See here for some power draw comparisons for hard drives:
Those are relatively recent, but they're all under 10w under extreme load. Let's be super-conservative and double it for slightly older hardware; for 3 drives, you're talking 60 watts. The SSD is much less, but let's call it 10 watts. Your CPU is going to be rated somewhere well south of 150 watts (unless you're still using a high-clock Prescott P4 or something.) If you're running anything recent, it will be under 105, probably under 75. PEAK. For gaming systems, your graphics card is going to be the biggest draw, and you probably want to base your PSU on your graphics needs entirely.
But don't take my word for it; here's Newegg's PSU calculator, which is HIGHLY conservative (they have incentive to sell you a bigger PSU, after all.)
I think you'll have a hard time coming up with a single-GPU desktop scenario where they recommend > 750 Watts. Again, that's a very conservative estimate, not necessarily for a quality manufacturer's PSU, so it's got a lot of fudge room built in.
HOWEVER, as noted above by the_Monk, the main factor is really not wattage. It's amperage, and mostly for the 12v rail. You want to make sure you have enough amps on whichever rail your devices are plugged into so that peak draw allows them to power on and not overload the rail. This is more a matter of buying a quality brand/design, rather than just looking at wattage.
To Gfirefly - if at all possible, try the drives either in another machine or with an external reader of some kind to separate the rest of the machine as a factor.
And I agree with the advice that a 7 year old PSU is likely due for replacement; at that age you're likely to start seeing things like capacitor failure. Note that this applies to your other components as well - and it can be very sudden. Or they could just keep on ticking indefinitely; the point is that you're on borrowed time and shouldn't be surprised when the failure happens.