I haven't looked at the current job opportunities but I'd imagine they'd all want you to be current on type with a MINIMUM of 500 hrs, most likely more...So you get a rated on 737/320 but your applications still going to get binned as you haven't got the hours. You're alienating yourself from the market you should be targeting by getting a rating. You would essentially be over qualified for the low hour cadet schemes as they would (I assume) want you to undertake a SSTR with their trainers/SOPs. I doubt they'd look at you if you already had a rating.
Secondly what happens if you get rated on a 737 and then get offered a job which requires you to undertake a SSTR for the 320? You'd take it so you've just thrown away money (a lot of money) on a rating that you've never used and most likely had no bearing in you getting the job in the first place.
So with your new rating you're going to have to get a job in the open market and as we all know there are many experienced and rated pilots out of work (sadly a couple of hundred more pilots from Spanair as of today) who you would be competing against so its pretty straight forward who they'd prefer to employ...
Lastly, lets say you've got your rating and still no job for a year - Are you honestly going to call it a day there? or are you going to be tempted to give it another shot by buying 500hrs? which by that stage the airlines might be wanting 1000 hours on type as an absolute minimum.
If I were you, I wouldn't.