I use a 25" LG and a 55" Hitachi both in 1920 x 1080 mode
They are cheap now. Although I don't do CAD, if I were working on a desk, I would use a two 25"'s in vertical or Portrait mode instead of traditional Landscape mode side x side., That way CAD could use both screens or you can read long webpages on one while long emails on another. Then side by side you get around 22x24" of work space.
I found any card will do as all you need is 32MB of VRAM to do 2D stuff, but I prefer MATROX which also make triple output cards and they are cheap 16MB surplus
MATROX on EBAY which are very fast and lean ( Even 8MB is enough for most 2D people) ( Wont support Aero)
You can also use the onboard VGA and the PCI VGA and HDMI for the other pair and set bios to use both. ( If you have onboard VGA) Video cards usually have adapters for VGA and Monitors these days still have both.
When the old LCD monitor flickers and changes tint in corners, you can also repair them with dexterity. Digikey has dozens of tube lengths usually in 2mm diam for laptops and desktop screens. Tubes Are ~$7 each so I bought extra for laptop at one time.
Always keep monitors at the minimum brightness that you can tolerate. Your eyes will thank you.
Most cheap monitors these days are still 96 dpi. Smart phones are almost 4x this now.
Next gen monitors now on the market support 4x as many pixels for twice the resolution in x,y so instead of 2megapix, ( 1920x1080) they are 8megapix. So watch out for those if you can afford.
Old CRTs were a little easier on the eyes because of the phosphor remnance. Even tho you cant see the flicker on moving LCD images at 60 or even 120, your eyes can sense it so 240+ are high end types, which can cause eye strain with high brightness. But for this application doesn't need it.so just FYI.