As far as PSUs are concerned, be informed. Before you buy any PSU read accurate, objective PSU reviews at reputable sites such as www.jonnyguru.com or www.hardwaresecrets.com on the EXACT model PSU that you are interested in as some brands have good and poor quality PSUs.
You can also get an accurate rating of how much PSU power is required for your current or future system at the PSU calculator link below. Once you know the total PSU watts required then you need to confirm that the 12v rail has enough amps. to support your Vid card(s) and the rest of the PC system.
There are several websites that show the Vid card power consumption in watts. Divide the watts by 12 to determine the amps. required on the 12v rail(s). Add 15 amps for the rest of the PC on the 12v rail and you now know the Minimum total 12v rail amps required under full load. It's best to have at least 5-10 amps. reserve on the 12v rail available under full load so the PSU is not loaded to 100%.
It's also worth noting that people often misunderstand the 80% power rating. This is a rating of the PSU's energy efficiency not it's output. 80% plus PSUs use less grid power to produce the same PC power. If it's 80% Bronze, Silver or Gold the cost savings on electricity is pretty small between Bronze, Silver and Gold unless you are paying very high rates for electricity so any 80% rated quality PSU is fine even if not Gold. For those who leave their PC on 24/7 a quality 80% PSU is a good investment.
Ahhh, so its old junk. That will help it last longer...
PSUs don't last forever. Use it if you want as it will power everything ok, but I wouldn't keep that ticking time bomb for long. Why risk your shiny new system to something that still has that little red voltage switch?
If you already have it well then I guess it'll be suitable.
But sometimes when crappy power supplies go out, and the risk goes way up when they are old, they can take out your new parts too.
If you check out the newegg email deals regularly you can pick up a good 400w psu for $20-30 After rebate or after promo code. I got my 380 for $29 after $10 rebate and $5 promo code. I also just picked up a Antec Ne0 620w for $30 after $20 rebate.