That might be a bit risky. I'm guessing that it's an "Allied" brand power supply. I have one of that same brand in My Core 2 Quad setup, but I'm not really pushing it. @ 450w it'll probably be able to handle the GTS 250, but I suggest that once you put it in, you should monitor the +12v voltage in Speedfan or HWMonitor:
http://www.almico.com/sfdownload.php
http://www.cpuid.com/softwares/hwmonitor.html
If it's under 12v, you'll probably want to get a new Power Supply.
		
 
		
	 
The +12V reading in those are useless, apparently my system doesn't even have a 12V output, a friend of mine, on an Antec Earthwatt's, has +0.3 on his 12V, the only way to get an accurate reading is to test it with a multi-meter
Besides, a cheap 450W unit that came with a case won't be powering a dedicated video card intended for gaming, which the 250 is. No, it doesn't draw a lot of power, but it is still more than the unit will be able to handle.
Before getting the video card, I would very highly recommend you get yourself a better power supply. If you can afford both, all the better 

 Unfortunately, I don't know any websites or shops in SA, so can't really recommend an option, but if you can find a Corsair, Antec, XFX, SeaSonic, Silverstone, Be Quiet! or Enermax unit over there that is 400W or more, that is what you should be looking at getting