You will definitely have a big loss in your gaming performance.
Now your monitor runs at 1366x768 resolution,that is 1049088pixels
, when you upgrade your monitor to a 1080p LCD, the resolution becomes 1920x1080=2073600 pixels
,which is twice the size of your 18.5" LCD monitor
.So as the resolution increases images become crisp,brighter,and more colourfull and icons become smaller. So you will decrease your graphics cards perfomance here @ big resolutions
Personally i like SAMSUNG LCD monitors,i use a samsung 2233sw full hd monitor,So i recomend you a SAMSUNG LCD,you can select it from HERE
according to your budget!
Buy a good lcd monitor having 1920 x 1080 resolution or more which is a future proof buy!