New
#11
The 550ti uses a single 6 pin PCI-E power connector, which is rated at 75W. The card could draw another 75W through the PCI-E X16 slot. According to this:
The Truth About Graphics Power Requirements V2 - Atomic 3.0
a 550ti actually consumes over 150W at full load. If all of that comes from the +12V supply, that'd be 12.5A. That leaves only 5.5A for the rest of the system, given your (rather poor) "500 W" PSU.
It looks like your power supply is not adequate for the 550ti.
If you can consider a replacement, you may wish to look at:
Newegg.com - CORSAIR Builder Series CX430 V2 (CMPSU-430CXV2) 430W ATX12V v2.3 80 PLUS Certified Active PFC Power Supply
(Corsair Builder CX430 V2) or:
Newegg.com - Antec NEO ECO 400C 400W Continuous Power ATX12V 2.3 / EPS12V 80 PLUS Certified Active PFC Power Supply
(Antec Neo Eco 400C). They both list for $45, but there's a rebate on the Corsair at the moment that takes it down to $25. (I dislike rebates, but I use them.)