let's say that I'm gonna buy a PC with this specifications:
CPU - Core i5-750
Memory - Kingston 2GB DDR3 1333Mhz
HDD - WD Black Edition 640GB 32MB Cache
Motherboard - Asus P7P55D
GPU - Geforce GTS 250 1GB
Does a Thermaltake TR2 RX 450W will be enough to handle the system?
And if not, how much watt are needed?
According to http://www.antec.outervision.com/PSUEngine it looks like 450W would be fine but I'd add in some room for if you want to overclock or a lot of fans or any other goodies you might want to have running also.
The general rule of thumb is a high quality 500 to 550 watt power supply with sufficient current (amps) on the +12 volt rail(s) can easily power a system with any single video card made. A high quality 700 to 750 watt power supply with sufficient current (amps) on the +12 volt rail(s) can power a system with two video cards operating in dual mode. There are a few exceptions like the new ATI Radeon HD 5XXX series cards which use less power due to their energy efficiency.
A high quality 500 to 550 watt psu will have a +12 volt rail rated at 40 amps. A high quality 700 to 750 watt psu will have a +12 volt rail rated at 60 amps.
In addition the power supply should be at least 80+ Bronze certified for energy efficiency. There are some models available which have achieved 80+ Silver and 80+ Gold Certifications.
Corsair, PC Power & Cooling, and Seasonic are some of the brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Some of the newer models come with a 7 year warranty. Lately we've been seeing a few other brands offering some high quality units. One example would be the Antec Earthwatts series which is a major improvement over Antec’s older psu’s like the Basiq models.
Here is a link to a technical review of the Thermaltake TR2 RX 450 watt power supply and comparison with other 450 watt power supplies:
I suggest you read the entire review and pay particular attention to the comparisons with other power supplies. It should become readily apparent the Thermaltake unit is not a stellar performer. Although it costs a little more the Corsair VX450 would be a more appropriate choice.