Depending on your power supply in the system already. You will need to remove the side panel of your system to get the make and model of it.

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this is my PC but has 2 TB not 3 TB

We'd need the exact model im afraid, as manufacturers like HP, Dell etc tend to use substandard PSU's in their systems to keep costs down. Even though they are rated for 460w, if they were actually loaded to that wattage they may not last very long.

Hence why it's important to direct you to a good supply to we can make sure your expensive hardware is safe.

well it doesn't even have any branded on it

It would probably be a good idea to replace it then, a 660Ti can run off a 450w watt supply, and in the UK the XFX 450w retails for about 35GBP so its not that much of a spend to get a quality investment, especially for a graphics card that expensive.

so the 660 ti will work on my motherboard, just need to get a new psu? would a 500w be okay ?

Yes exactly. Just make sure you choose a quality PSU, i could recommend a couple, where are you based?

Im from the UK and so i do i no what my motherboard can take ?

In that case the XFX 450w is the cheapest quality unit in the UK i believe. What do you mean what can it take?

how do i know how many slots its has and stuff and what the total watts it can take