The only symptom I'm aware of is total system failure under load, i.e. completely crashing when you try to do anything significant. When you try to draw too much power, things don't just slow down, they MELT down.
Just checking the specs of your equipment though, it looks like the CPU and GPU are using more than half of your PSU wattage. If you intend on upgrading your GPU any time soon, you will almost certainly need a new PSU as well, and 'top of the line' is affordable for those - ~$100 for a 1000W PSU which will probably be more than enough for you for a long time.
CORSAIR CMPSU-650TX 650W ATX12V / EPS12V SLI Ready CrossFire Ready 80 PLUS Certified for $70 after MIR and an upgrade to EVGA 512-P3-1153-TR GeForce GTS 250 512MB 256-bit DDR3 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card for about $132 with shipping. These would be all the upgrades you would need for your system to ensure longevity and stability.