Im running atm 2 PCI cards (Wlan Card and Tv Tuner) and I have a free PCI-Express slot. I believe I've got a 250watt Deltra Electronics PSU, which has been in there since day1, and I've written some PSU details below as seen on sticker:
+3.3V & +5V Total Output Power Cannot Exceed 110W
+3.3V & + 5V & +12V Total Output Power Cannot Exceed 245W
From my brief research, I know that 250w PSU is not powerful enough for the better graphics cards on the market. I also believe that AMD/Nvidia stated PSU requirements are largely overstated due to being on the safe side. Hence, with a budget of £50, and by reading reviews online, I've come to the conclusion that this card might be the one for me?
I would save for a better card then the HD6450, But the card will work with the PSU, the card wont even use over 50watts if that. So you should be more then fine to run the card. But I would save for something better along with a new PSU.