The Apevia ASPIRE 600W power supply is not a 600 Watt power supply. It's realistically only a 500 Watt PSU (jonnyGURU).
Its combined +12 Volt continuous current rating is anywhere between 26 Amps to 30 Amps.
It has two 6-pin PCI-Express supplementary power connectors.
For a system running with a single GeForce GTS 450 NVIDIA specifies a minimum of a 400 Watt or greater power supply with a combined +12 Volt continuous current rating of 22 Amps or greater and with at least one 6-pin PCI Express supplementary power connector.
Your 600W PSU should be sufficient to power your system with a single GeForce GTS 450.
An overclocked GeForce GTS 450, itself, will draw 8.7 Amps from the +12 Volt rails under a realistic full graphics processing load. If you run FurMark it will draw 12.3 Amps from the +12 Volt rails.