My situation is that I have IBM T221 and a GTX 590, however, it didn`t support mosaic mode so that cannot give me 3840x2400 decently, thus I had to buy another ATI card which has analogous feature.
Don't know what you're talking about, I've been running a T221 successfully at seamless single desktop resolution of 3840x2400 with Nvidia cards from 8800GT to various 4xx series cards, a GTX580 and a GTX680, before and after quadrifying the cards without any issues at all. No modification is required to get Nvidia cards working perfectly with T221 monitors, at least in XPx64 and Linux (the only two OS-es I use).
My experience with ATI cards and T221 has been utterly dire. The most recent hardware I've had moderate success with on T221 has been the Radeon 4xxx series cards, and windows drivers of that approximate generation (2011). This is because of the following:
1) No ATI cards I have ever found that are later than 4xxx series have more than a single DL-DVI output. If you are using DL-DVI->LFH60 adapters (google for Cirthix T221 and you'll find it) you need 2x DL-DVI ports.
2) If you are using 4x SL-DVI ports, you are even more out of luck because no ATI card comes with quad DVI ports. Nvidia cards don't either. You could potentially get away with running two cards on a DG5/DGP T221, but DG1 and DG3 require genlocked outputs (allegedly - never tested this myself with multiple cards on my DG3 - perhaps I should) which won't be the case with outputs from 2 cards.
3) If you are using 2xSL-DVI, and you want more than 25Hz, you won't be able to achieve it with ATI cards because any recent ATI driver will flat out refuse to do any mode except what the monitor reports via EDID. The driver will even ignore custom monitor .inf drivers. If you use an Nvidia card (or use the open source radeon Xorg driver on Linux which supports custom modelines) you can actually achieve about 33Hz with 2x SL-DVI outputs.
4) Nvidia cards happily reliably push 165MHz on each SL-DVI output (maximum standard supports). My Radeon 4850 produces strange blue artifacts when pushing 165MHz, I had to drop it down to 160MHz.
5) ATI cards produce pretty obvious tearing between the two halves of the screen. This is obvious in all applications (e.g. dragging an application window around the center of the screen, very obvious in full screen games at 3840x2400 or when playing back HD videos). Nvidia cards do not suffer from this - they seem to use a single large framebuffer so there are no sync issues.
It is all these issues with ATI cards that a T221makes particularly obvious that are the main reason why I consider ATI cards unfit for purpose.
The only reason I am using a 4850 at all is because I have a slightly broken motherboard PCIe slot that seems to have a failed PCIe lane #1. ATI cards seem to be clever enough to go into x8 mode and use the 2nd 8 lanes while all my other cards (GPU and otherwise) don't do that and thus don't detect in that slot. This is
the only good thing I can say about the ATI cards.