Hard to say with an unbranded PSU. 450W is easily enough for that graphics card in a normal system, but the question is whether that unbranded PSU can really deliver 450W (or anywhere close to it) reliably.
If your running a dual core system it should be fine.
Your system won't draw anywhere near 450W. Not even 300W. So there's a decent chance it'll work alright. But a noname PSU is never truly safe, so I'd recommend replacing it with a reliable brand PSU at some point.
grab a radeon hd 7750 .its better than gt640...and will work with your psu for sure .and it will work good with your rig....