Upgrading my single slot graphics card

If there is nothing in the slot underneath the PCI-E x16 slot on your motherboard(or nothing you can't move) then it shouldn't be a problem. The better question is whether or not your power supply can handle the new card.
Is the a reason you want that GTX 550 Ti in particular? It isn't a very good deal for the price.

You would need to check if you can fit a dual slot card and make sure you have a power supply that is able to sufficiently power the new card.

There is another slot, and beneath it is says PCI2, and my power supply can handle it, also is there a better card than the 550 ti for around $150?

Yes, there should be a slot, what matters is that there is nothing in it because the cooler on a 2 slot card will be blocking it.
The best cards for around $150 are the HD6850 and GTX 460.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
I'd get that GTX 460 if you can afford it. It's got great cooling and a large factory overclock and can actually go much higher if you want to.

And if you don't mind me asking, what's the difference between pci express 2.0 and 2.1? and how can you tell which one you have

technically there's no difference between PCIE 2.1 and PCIE 2.0 standards both are the same, from Wikipedia
" PCI Express 2.1 supports a large proportion of the management, support, and troubleshooting systems planned for full implementation in PCI Express 3.0. However, the speed is the same as PCI Express 2.0. Most motherboards sold currently come with PCI Express 2.1 connectors. "

+1 for the GTX 460