Hey man,
You REALLy need a new PSU, and even though your system probably doesnt need this kinda power, its always better to get a stronger PSU.
Ideally, I'd recommend a Seasonic PSU , but even Corsair is great, and this seems brilliant for price. Seasonic is basically OEM Corsair
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139028
And a 550ti isnt really money/performance, an HD6770 comes close to it, if youre ready to shell out maybe 30-40$ more, you can get a BEAST, namely, the HD6870.
Again, i wouldnt really recommend last generation cards, but man this ones a BEAST.
If you dont want to get this, another VERY viable option is the HD7770(
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102948 ), same price as 550ti. Plus the TDP of a 550ti is 116W, while that of a 7770 is 80W. And i think, you JUST MAY be able to pull this card off with your PSU, but DEFINITELY upgrade in the future.
http://www.anandtech.com/bench/Product/536?vs=541
You can also see the HD6870 vs 550ti
http://www.anandtech.com/bench/Product/540?vs=541
And 7770 vs 6870
http://www.anandtech.com/bench/Product/540?vs=536
So my final recommendation - the PSU and HD6870 , i REALLY think thats a great investment.
7770 is also nice, but just see the comparison between the 7770 and 6870, and a small gap of ONLY 40$ (or so).
Youre anyways gonna need a PSU upgrade soon, if you want a stable system.
Last thing i want toa ddress- the bottleneck? Well, i guess it might be the slightly low powered CPU, but if you think youre up to it, you can get a cooler, and overclock.
http://www.newegg.com/Product/Product.aspx?Item=N82E16835103065
This is cheap and from what ive read, BRILLIANT.