hi mate. Well the 19" market is certainly there with excellent gaming screens. That sector has begun to get quite saturdated really with fast TN Film based panels. models like the Viewsonic VX922, Viewsonic VX924, Samsung 930BF, Hyundai Q90U etc all offer some impressive responsiveness and excellent gaming 'feel'. The resolution is also low enough that you can turn up all the eye candy on modern graphics cards and the TN Film technology is really well suited to fast transitions and to gaming. If you look in that article linked above i only wrote that very recently and it covers all the best gaming screens. The main issue with the fast 19" market is that you are sacrificing in other areas (viewing angles and movies mainly). However, purely for gaming, these are excellent choices and well priced as well. Certainly no point in getting a 17" imo since the extra size of 19" screens is really nice and offers improved immersion in games. If it were me, and i was looking for a pure gaming screen, the Viewsonic VX922 would probably get my vote. If it wanted a more all round screen, i would probably look at models like the Viewsonic VP930. However, they aren't QUITE as fast for gaming, and more expensive, so depends what you're looking for.
The 20" market is beginning to show massive improvements in responsiveness and the panel technologies used (P-MVA, AS-IPS mainly) really offer more all round performance than TN Film panels. However, they are mostly in a similar position to models like the Viewsonic VP930. Nice and responsive, certainly fine for gaming, but just not QUITE as fast as the 19" TN Film models with heavy doses of overdrive. The Belinea 102035W and Viewsonic VX2025WM are effectively larger versions of the Viewsonic VP930 covered in that article. If you want a much more all round performance then these are certainly more suited.
A couple of things to consider, not all games support WS resoltuion and only some of the models allow 1:1 pixel matching to allow you to play with black borders at a native resolution. this might be an issue on some screens. also the extra graphics power needed to run at a higher res on a 20" screen may be an issue. something to consider
although the 20" market is using mainly non TN Film panels, the one model which is so far reported to offer TN Film like performance in gaming is the NEC LCD20WGX2. This might be a good option for a gamer, while also offering all round performance in other areas which TN models cant offer. If you have the graphics power to power the higher res and have the budget for a 20" WS this would be my choice at the moment for a gamers screen in this sector. However, i am still a little undecided whether it overtakes the 19" models like the VX922 in terms of the most ideal gaming TFT