thinking of doin' a practice race before my sprint

By all means, go ahead! Sprints are intense but not long so recovery time from them is relatively short. It's definitley a good idea to practice everything beforehand, a race simulation if you will.

In regards to the time on the spin bike, 120-160 rpm? Ah, wow! That's super incredibly high. How about backing that down to about 90-100 rpm instead. It's really hard to say how much time you should spend on the spinner at a certain cadence without knowing the load/speed. A typical sprint bike segment will last on average about 45 minutes (give or take depending on your speed) so hopefully that will give you some guidelines to go by.

As for what to do the week of the race, do light workouts, stay off the strength training and make sure you have a day off the day before your rest. Oh and sleep. Sleep is good. It'll recharge your batteries and help ensure that you're ready to go come race day.

Best of luck!

120-160 cadence?!!!  Wow,  120 is my max cadence and I can't sustain it for very long.  I echo Caius' suggestion to back it down to 90.  Depending on conditions, most can finish a Sprint distance tri in about an hour and a half.  This one is indoors?  So terrain won't be an issue.  Is the bike duration the only thing you're trying to figure out? 

The best thing about a "dry run" is it gives you an opportunity to practice transitions.  I race with a girl that is about my speed but I manage to beat her transition times every race.  Why? Because I practice them.  And I make note of what worked and what didn't. 

Time goals for a first-time tri are tricky. My advice would be to go more by the level of effort you want to sustain and let the time fall where it falls.

As for cadence and speed, there no correlation without knowing what gearing you are using. Assuming a flat surface, at 120 rpm in an easy gear you could go 10 mph. 90 rpm in a hard gear and you're going over 20 mph. No way to know.

My first race advice: Relax, practice transitions from swim to bike and bike to run. Do it all together or split it up if you like. You won't gain any significant fitness between now and an early December race, so what you will benefit from is mental preparation and the transitions. If a practice race does that for you, go for it. After your first race, make a note of what went as expected and what took you by surprise and then start planning for your next race.

Yeah I was wondering about that...

HR is just as relative... variable from one person to the next and different when conditions change. My effort at a HR of 120 on a flat road with no wind will be around 18 to 19 mph. At 160 bpm, same conditions, I'll be going 24 to 25 mph. Three years ago when I started training, at 160 bpm I'd be doing around 16 mph on my mountain bike... It's all relative.

Ok well I'm guessing about cadence, 120 bpm is something I'm familliar with (I was a Band geek in highschool), and I'm just trying to translate to spin bike....

AND I forgot that higher gear = slower cadence but faster speed

Yup this is me, ROOKIE-FIED thanks y'all I really appreciate it.