Having done Katowice - Warsaw last year I'd recommend traveling on either an EC or an EIC train.
On the outward leg I traveled on an EIC (Express InterCity) service. The majority of coaches are compartment coaches, and are clean, very comfortable and air conditioned. Even in 2nd class you are still offered a complementary soft drink!
Both EIC and EC trains are reservation only but I think you can get away with purchasing the reservation on board. I used an InterRail 3 day pass and obtained the reservation for the Katowice - Warsaw Central leg from Katowice ticket office (note: use a phrasebook or write down your request since the ticket clerks don't speak much English). However, when on the train, I was told the reservation, which was free, had been issued for free by mistake and I'd have to pay a 12 zloty surcharge.
On the return leg I used an EC service (EC 110 PRAHA) and paid for the reservation on board. Like the EIC service, this train had a restaurant car, and was formed of mainly compartment coaches, and was just as comfortable as most EIC services.
Bear in mind that if you do travel on a TLK service, the quality of the coaches used can vary considerably. Sometimes, you may travel in an air conditioned EIC coach with powerpoints for every seat, or you may end up traveling in an unrefurbished 1970s coach with no air conditioning.
Finally, regardless of whatever train you catch, DO NOT travel via Czestochowa since your journey will be a lot longer.
Hope this helps
