wich fan for C2D oc ??

I'd take the ASUS Silent Knight - I think it would look the coolest sitting on top of my TV.

...or did you have some other requirements in mind?

Which C2D?
How much OC?
What case do you have?
How much do you plan to spend?
Is noise level important?
Is weight important? (do you plan to carry the computer to LAN parties?)
Is mounting mechanism important?
Do you want a replaceable fan?
Do you want the best cooling performance you can get, at any cost? (without regard to any of the other questions above ?)

...the more information you can give about what your paritcular setup is and what your priorities are, the easier it will be to answer your question with something besides the typical:

"Get an Ultra 120 Extreme - it rox"
"Tuniq Tower 120 FTW!!"

etc.

Ok, that's a bit more to go on ...

Your 3.4GHz is on the upper end of overclocks for a E6300, but the amount of heat being generated will depend more on the amount you had to bump the core voltage up than the actual speed achieved (FSB * cpu multiplier). If you're still using the stock Intel heatsink and it's not crashing or throttling itself back all the time, chances are that you didn't have to add too much voltage to get there (you must have a pretty decent chip and mb).

With the above in mind, every heatsink you listed would be better than the stock Intel heatsink, so here's some notes/opinions on the ones you listed (you might want to have a look at this recent cooler roundup)...

asus silent knight:
- I hadn't paid a lot of attention to this one (didn't get a lot of reviews), but if I recall correctly, it performs well enough, but may be pricey.

scythe infinity:
- replaceable (clip on) fans
- very large
- and heavy
- yet still uses intel push-pins... I personally wouldn't be comfortable with that much weight on that mounting mechanism
- can be relatively expensive
- performance is good, but there are lighter, smaller, less expensive alternatives that might suit your needs

scythe infinity ninja:
- replaceable (clip on) fans
- can be found online for ~$35 or so.
- all around decent cooler with good performance (better than most, when using a low-speed fan for silence, due to the wide fin spacing)

theres an ocz one wich is almost the same as the scythe:
- similar in most regards to Ninja, though possibly a degree or three worse performance.
- price/avialability/aesthetics might make it worthwhile over the Ninja, but that's about it.

artic freezer 7 pro:
- excellent bang-for-buck performance.
- small relative size and weight
- easy (Intel push-pin) installation
- can often be found as low as ~$20-$23 online

coolermaster hyper tx:
- excellent bang-for-buck performance.
- as far as I can tell, nearly identical performance to the ACF7Pro, above
- but with a slower rotating/quieter/larger-bladed fan
- shroud diverts some air to help cool MOSFETs
- small relative size and weight
- easy (Intel push-pin) installation
- can often be found as low as ~$24-$28 online
* Usually a few bucks more expensive than the ACF7Pro above, but I chose this one for my system, over the ACF7Pro, for some of the reasons above and am very happy with it.

zalman 9700:
- very good performance (usually found within the top 3-5 air-coolers on the market)
- requires removal of motherboard for mounting
- fairly loud at top fan speed
- price is about 2x is much and noise is about 2x as much as Hyper Tx, and only cools a few C better.

Personally, some of the low-end, ready-made water cooling systems look interesting, but common opinion seems to be not to bother, since they don't really perform any better than higher end air cooling and add a level of complexity/hassle.

Basically, if you're going to move to water-cooling, most folks recommend spending upwards of ~$200-$250 for a nicer system to start getting any real benefit. If you've got money to burn (so to speak), the low-end systems might be something to get your feet wet (so to speak), but you'd likely end up replacing it relatively faster than a higher end WC system, or a nicer air system.

BTW, you didn't list some of the higher end air coolers, so I didn't comment on them. There are several coolers that will perform better than the bang-for-buck coolers when you start adding a lot of juice to the cpu (Tuniq Tower, Ultra 120 / extreme, etc.), but they are more expensive, heavier, larger, and maybe louder and generally requre removing the mb for installation.

For modest to moderate overclocking, the ACF7Pro, Hyper Tx and some others like the Scythe Mine and Scythe Ninja (in increasing cost, but not necessarily better performance ordering) can all perform well enough, with typically less hassle.

BTW, I just noticed that Newegg has the Hyper Tx 2 for:

$26.99
($11.99 after $15.00 Mail-In Rebate)
$5.84 3-day shipping

...works out to under $18.00 shipped, if you don't have to pay taxes and manage to get the rebate (expires 7/15/2007). I'm not crazy about relying on rebates myself, but that would be a heck of a deal.