The one 1" slot is next to it, or above/below the PCI-E x16? If above/below it's a PCI-E x1, basically a higher bandwidth PCI. Only slots need to look at in the pic are the one the card is in, and the one that is 1" long under it (Is above really, but upside-down pic), those are the PCI-E slots.
As for the PSU, 300W is low, but what you'll want to look at is the #A listed under the 12V, not that the Wattage isn't important.
Should read something like: Numbers will vary of course.
<BrandName>
<Input>
<Output> 5V 3.3V 12V 12V2 (if applicable) -12V ~~
<Current> 30A 20A 18A 18A ~~~ ~~~
<Power> | 150W |
<Power> | 675W | 25W |
What you need to look at is the "A" number under the 12V Rail(s) and if those total are under 30A the PSU won't do well with new Video Cards.
Also, a PSU won't be cheap normally, if it's cheap it's likely off-brand and not one you want.