Like others have said I would work out what is wrong before you drive the car any more, you could destroy the gearbox.
One thing it might be and is something you can check yourself, if you have a decent jack and blocks or stands to make the car safe. On the output end of the gearbox is the 'flexi coupling' its a black rubber donut sandwiched by two flanges.
The coupling wears over time and the heads of the bolts can bash together and make a nasty grinding sound, it starts off occurring when you accelerate hard. Then it will become more prevalent as the coupling degrades more and more. Jack up the car (Make it safe!! and get someone to give you a hand who has some knowledge on doing it properly). Crawl underneath and where the gearbox is look, for the coupling and look at the bolt heads and nuts, see if they are bashed and shinny.
I hope it is only this as its a reasonably easy and cheep problem to sort. I was 17 years old when I took on my dads 73 2002 Targa (was the coolest car in the college car park , I'm now 27 and still have the car. I have done all the work on it myself in the last 10 years. Get stuck in and you will learn fast. The Haynes manuals aren't easy to follow and are at best a guide, imo.
Good luck
Ben