I'm interested in buying the project Perspex 6 but am aware it doesn't come with a cartridge so my first questions is what would be the best one to get. I also have a Onkyo AV TX NR838, I was hoping to plug my turntable into this. It does have a phono input, but would I need a seperate preamp. Your help on this would be great.
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I looked at a picture of your receiver and it shows a phono input on the back. It will need to be a moving magnet or a high output moving coil design, for it to work with your Onkyo.
If I may ask - why do you only consider Pro-ject turntable?
Any $100 MM would do, I wouldn't buy $800-$1000 cartridge at this point unless you intend to get phono stage soon.
While the Perspex 6 is a fine table with a tonearm capable of a $800-$1000 cartridge, I agree with the others and stick with a $100-$150 entry level MM cartridge at this time. Biggest reason being your electronics - your using the on-board phono stage in an AVR; electronics not exactly known for their analog performance. You would never get the most of a higher dollar cartridge.

Also, the Perspex's 9CC EVO tone arm is very low mass at only 8 grams so it should be matched with a high compliance cartridge. Ortofons and Sumikos come to mind. You might want to start with an Ortofon 2M Red at $99. You can upgrade the cartridge to three higher levels by changing the stylus, all the way up to the 2M Black at $750 that's sports a Shibata stylus. That is of course once you get away from the AVR.
Whether it be Ortofon, Sumiko or Grado, there will still be a difference, though I don't know how big a difference.
You could talk to people at needledoctor.com
I once asked them a few questions about cartridges, and they were straightforward enough. I didn't follow the advice but because of cost not because I disagreed. They know Pro-Jects, and they will most likely mount and align the cartridge for free if you buy both from them.
Grado hum with some tables, by the way.
Excellent guidance in the above posts. Cartridge sonics are a matter of personal taste. Since we know nothing of yours, it would have been presumptuous for anyone to jump in with, "Buy a ____, best cartridge ever!". Glad to see nobody went down that path.

OTOH, any phono cartridge, whether a $100 MM or a $10,000 LOMC, will only perform its best when paired with compatibile components. The two most closely related components are:
1) the tonearm (which provides the mechanical connection to the system); and
2) the phono stage (which provides the electronic connection to the system).

The posters above correctly addressed these components and their impact on your decision. This forum continues to provide guidance that's a cut above...

Thanks so much for the great advice and so sorry for the late reply. This does seem the best site to get advice. Over the past few months I have been looking hard and have now got the Clear audio concept wood MC. I have the CA nano phono stage also. I thought long and hard about what you said and decided to take a look at OK at other brands. For what I see all the components are evenly matched. Agin some of the comments I would never have thought of. As you can tell I'm new to the game. Thanks again 
The Pro-ject 6 Perspex can now come bundled with a Sumiko Bluepoint EVO III HOMC, not too surprising since Sumiko distributes Pro-Ject in the US. Still, I also concur with the previous comments because the phono stage is really where it’s at before you can completely hear what you’re paying for in cartridges.

Bundled Pro-ject 6 Perspex
I purchased the  Perspex 6 a couple months ago.  I got the package deal with the Sumiko Bluepoint evo III.  It is an amazing combo.  Just starting to get broken in and sounding better every day.  Just be careful with the cartridge if you go that route, it is very delicate and exposed.  Sounds amazing though.