Your power supply is fine for the 550Ti. Dell PSUs are decent, and this one is adequate to power most single GPUs.
How much money are you willing to spend on a graphics card? If you have $160-170 the Radeon HD 6870 is available for that price and is considerably faster than the 550Ti. At $150 the 6850 is available, which is also a good deal faster than the 550Ti.
Around the $150 price point there is also the 7770, however, I wouldn't recommend that one unless you are really comfortable with overclocking, the 7770's stock performance is rather lackluster for its price point, with only the rather high overclocking potential really saving it.
$200 is the most I would like to spend after tax so anything in the $150-$170 range before tax, but I don't think my 460 watt PSU meets the minimum requirements for those beast cards you mentioned or can it
A Dell 460 Watt is definitely sufficient for a 6870 or GTX 560, either of which would hit the upper limit of your budget. AMD and Nvidia overstate their power supply recommendations because there are a lot of really bad quality power supplies out there that can't deliver what they promise. The 6870 and 560 use around 150 Watts at most on their own. Also check your power supply and see how many 6 pin PCI-E connectors you have. The 6870 and 560 require two, if you only have one, you may have to get a 4 pin molex to 6 pin PCI-E adapter. Alternatively you can go down to the 6850, which only needs one 6 pin connector, though I would definitely go with the 6870 if you can afford it.
The only other thing I would worry about depending on your computer's case might be video card length. Some of the 6870s can be quite long, so if you have a smaller case you may want to open up your computer and measure how much distance you have from the back panel where your current video card is to the hard drive cage to make sure you can fit some of the longer cards. Card length is listed under dimensions on most online retailers. Just make sure you have sufficient clearance to get the card into your case.