The HSK is aimed at non-Chinese and ethnic minorities studying Chinese within China. There's an old version and a new version, the new version is slightly easier than the old version. It tests reading, writing, listening, and speaking (you have to make a recording). It's similar to TOEFL in English, and it's a requirement for entering some degree courses in China for foreigners to have an intermediate or advanced level pass. Wikipedia: Hanyu Shuiping Kaoshi - Wikipedia, the free encyclopedia has some more details.
A pass of HSK at advanced level would be equivalent to functional Chinese, the bare minimum you'd need in a professional setting, but probably not enough for most jobs as it doesn't test professional vocabulary and even at advanced level the fluency required when speaking for example is limited. The exam takes an hour or two depending on the level.
Baidu has more details on the PSC: http://baike.baidu.com/view/21049.htm, the information below is mostly from there. PSC is a spoken test aimed mostly at people whose native language is Mandarin, but also at foreigners who have passed the HSK. It's only about 15 minutes long.
70% of the PSC test is pronunciation and ability to read out loud, including reading out single and double characters, testing correct use of 儿化 and 轻声, as well as a 400 character passage read aloud where fluency is tested. The remaining 30% is an ad-lib on a randomly selected topic. There are three grades, with upper and lower levels at each grade.
The three grades represent different levels of spoken Mandarin appropriate for different jobs: broadcasters, TV hosts, and TV drama actors must have Mandarin equivalent to the top grade; teachers and university students must have Mandarin equivalent to the second grade; people working for the state must have Mandarin equivalent to the third grade.
For professional use HSK advanced followed by grade 2 or above PSC would be good goals to aim for. Really though the important thing is how confidently you can communicate with the language if you think you will use it in professional settings.