400W too small for i7 950?

A GT240 only needs about 5 amps. The processor will pull about 10 amps. That doesn't leave much for the rest of the system.

Your PSU is an older one with a relatively light 19 amp 12 volt rail. In your case, it's not the power rating that is important, it's the 12 volt output. Your system will be running the 12 volt rail right at around 100% of capacity.

If you had something like the now discontinued Corsair 400CX with it's 30 amp 12 volt rail, I'd say, "No problem." Even the not quite as good 430CX (28 amps@ 12 volts) would do nicely.

Considering the combination of an old, obsolete, not particularly reliable design with a light 12 volt rail, I'd say that you should budget for a new PSU. A 400 -450 watt PSU will do, but it needs to be a good PSU.

 
The following current draw from the +12 Volt rail(s) is what you will expect to encounter with your components:

Intel Core i7-950 CPU @ 3.06 GHz : 8 Amps under full load

Motherboard & RAM : 1.25 Amps

GeForce GT 240 Graphics Card : 5.75 Amps Maximum

Western Digital 2 TB Caviar Green HDD : 0.6 Amps surge, 0.24 Amps nominal

Lite-On IHAS424-98 : 2.5 Amps

That's a total of 18.1 Amps from the +12 Volt rail required for your components. We still haven't included any case & CPU cooling fans.

Your Cooler Master RS-430-PMSR/P Power Supply Unit is only capable of providing 19 Amps on its +12 Volt rail.

No doubt, you need a new power supply unit.
 
The Coolermaster may be labeled to be able to just provide enough, but unfortunately that line of PSUs is overrated garbage, so it will blow chunks, possibly damaging other components. If the 650W Earthwatts seems excessive (despite the excellent price), for example if you will never install a beefier GPU for gaming, check out the 380W Earthwatts for $45 ($30 if you play the rebate game and win).