1 gb graphics card for 300 watt psu

I'm confused... are you getting a GT 520 or not? If you are, what is your question?

to play BF 3,you need a complete system upgrade,your barely even to min requirements.Xp is old and should be layed to rest,your psu is to small,dual core barely cuts it anymore,Just time for a new build.

You do not "need" a complete system upgrade to play those games.
There are a couple of low-profile HD6670 cards that will run on a 300W or 400W (which is it?) PSU. You'll be able to play those games with this card, although you will likely be limited to mostly medium settings (a few might be high, but no AA).
Here is one: http://www.newegg.com/Product/Product.aspx?Item=N82E168... but note that it is wider than one slot. If that won't fit, this one will: http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Edit: Incidentally, a GT520 is not suitable for games, even though it is stronger than your HD5450.

I disagree with this system and a 6670 being able to adeqately run BF3. I have core 2 duo 2.66 with 4GB RAM and a 6770 and it *barely* runs BF3 at low resolution with all settings turned all the way down. The stuttering is terrible. You will be OK in the other games but BF3 will be painful.

If you actually "read"what I said,I said to play BF3 only.to tell someone that a 3-400 watt psu is fine without knowing anything about it(brand,age specs) is also poor advice.His current system barely meets Min requirements as it is.

Not to mention he's running an ancient OS,which will not take full advantage of newer hardware and games.

get the ATI 7750 , overclock ur processor to 3.5 ghz+ and upgrade to win 7 and ofc dont go beyond 720p

He needs a low-profile card, and I don't think there's a LP version of the HD7750 yet.

I thought BF3 wouldn't run in DX9? If so, he can't play without changing his OS, regardless of his hardware.