Computer power supply design

IMHO, a rating is probaly for the supply as a whole.

A fully loaded ATX connector is limited to 8A per pin (regardless of voltage (ground, 3.3, 5 or 12v).

You'll also need to be aware of the wire gauge from the board to the connector...

And, finally, there is a max current listed on the PSU for each voltage (and larger supplies that have two 12v "channels").

As others say, the power rating is for the total of all outputs.

Each voltage rail will also have a specific maximum current rating. Some supplies list those on the label.

Tip: look for ones with an "80+" rating, the higher the better - bronze is the basic one, gold is the top.
Gold class ones can be 90% efficient or higher, and the higher efficiency the less waste heat the produce.

Some cheap non-80+ types are around 60%, so around a third of the input power is lost as heat.

If the 180W supply is doing what you want, stick with it. One factor about computer PSU is that they all have an optimum efficiency range, usually in the 70-90% of full load. If running at half load, your efficiency "usually" is less, thus producing more heat internally. Thus, unless you have a reason to go to a 240W or 360W PSU (like adding video card), I would stick with the 180W supply. If replacing a 180W supply with similar, then definitely look for a high rated efficiency one.
For example, the Dell 3010 SFF supplies come in two flavors, a 240W high efficiency 87-90% at 50%-100% load, or a cheaper 240W "standard" PSU at 65% efficiency.
Looking at the Dell 3050 SFF specs, there are 2 possible PSUs. One is 180W PSU at 85% efficient (80 Plus Bronze). The other is also 180W but at 92% efficient (80 Plus Platinum). I see no reason to change either of those just for the sake of changing...
Also remember that a 240W PSU for a mini tower will not fit the SFF computer...