thank you Plekto I'll look a review of that one !
Czornyj I am having an EIZO CG243W-BK
I'd add another high bit LUT IPS with the same panel height - NEC 2490WUXi2 or 2190UXi. Or at least high bit LUT S-PVA - Eizo S2433W or NEC 2190UXp.
A perfect companion for CG243W would be Eizo SX2461W - it is limited incarnation of CG243W, without the posibility of hardware calibration, so you can't use Color Navigator to calibrate it, but the panel and electronics are basically the same (H-IPS with 12/16bit 3D LUT, 10bit displayport, electronic uniformity compensation and brightness stabilization, and everything you ever wanted to have in a display, but were afraid to ask), so it's almost in the same league.
I'd rather recommend not to buy a chepeo LCD. CG243W is one of the finest panels on the market and if you'll get some poor quality display, it may be difficult to get at least similar look - that might be irritating, could spoil the comfort of work, and even might have some influence on the way you see colors on your CG. I also think that it's convinient when the second panel has same vertical size and resolution, so 21" or 24" should be the best match.