What are you lookig for in this rifle? The two .338 cartridges you mentioned are for long range work and a .375 H&H is usually around a 300 yard cartridge. I agree with Jimro about the .338 Win Mag is plenty, and in most cases so is a .300 Win Mag.
If you are looking for a butt kicking magnum round that isn't going to break the bank on purchasing a rifle, look for a used 8mm Rem Mag. Cartridges might be a problem to find but there is a lot of data if you reload. You can usually pick up a nice used Remington 700 for around $800.