donno how the prices are in england, but here (US) it would be hard to find a 21+ for 170 unless you are going online. You also need to consider if you game with it or just general computer use or watch movies on it.... or if u need it to look good...
cheapest one i found on newegg was 159.99 for a 22 inch (21's are more expensive)
it refers to the response time, how fast the pixels can change their state, nowadays it's usually a grey-to-grey transition. the smaller the better. with fast response there won't be much ghosting etc.. Some models have as low as 2ms response time but 5 is still very good. I have an old 17'' viewsonic with 16ms response time and it's still good enough (but I think it's actually on-off cycle response time, it used to be measured like that back in the day...)
That display is based on TN-panel, TNs are generally very fast but little poor on colour reproduction and viewing angles when compared to other (more expensive) panels such as different kind of PVA and IPS-types. But that won't really matter if don't do any professional image editing or other graphicall work on it.
I think the biggest plus for that acer is that it has DVI connector, most cheap lcds only come with analog vga connector, it affects the picture quality some what and makes the calibration process more diffucult. With dvi you'll get sharper image with no fuss what so ever...
What resolution do you run your 19" CRT at? if its normally running over 1600x1200, then you really need to look at a LCD in the Flesh before deciding what to buy... If your CRT was a fairly low end low resolution model then you will be fine if your CRT previously ran at high resolution (some 19"crts could run at 2048x1536) then you may well have problems with the physical pixel size of a 22" widescreen LCD, Personally I HATE my 20" (same "resolution" \ smaller pixels than the 22") LCD monitor, so much so that Ive relegated it to second monitor status and put my old CRT back for gaming use...
no amount of AA can ever cure jaggies on a LCD screen, a CRT screen at high resolution you can completely eliminate jaggies with relatively little AA...
Also worth making sure you are happy with viewing angle issues, fast response rate LCD screens can have pretty awfull viewing angle issues where it can be hard to find an angle where you dont have an area of the screen looking "wrong" makes for terrible problems in already dark areas of games. Ive got two of these LCD monitors :-
My wife likes them because they look "stylish" on the desktop, personally I think desktop use and web browsing is all they are any good for...
If on the other hand you run your CRT at 1280x1024 then go for a LCD, you will probably find the image quality an improvement! You really need to get a look at some screens before you make your purchase
AA = Anti Aliasing, its a graphics option that attempts to reduced jagged edges to objects to make edges less "stepped" etc
Take a look at this image to see the "staircase" effect on the cape:- this is the jagged edge we look to reduce.
What anti aliasing does is to try to try to re draw a low resolution image to try to look as it would if it were rendered at high resolution. by choosing to fill different pixels etc however the smallest element it can fill down to is a single physical pixel, which is where pixel pitch and type comes into play.
Ok now to show how pixel pitch can effect it heres a comparitive scale example of how the physical size of a pixel can effect the appearance of these jagged edges.
first we have a pair of diagonals made up to represent .40 pixel pitch and next we have the same diagonals made up to represent .20 pixel pitch. You can see how the edges of the physical pixels are so much larger and more noticeable on the 0.40. This is where big LCD screens fail... 0.20 is around the pixel pitch of a high end 19" CRT running at 2048x1536 - CRTs have an ability to shrink or grow the pixels with varying resolution, running that 19" monitor at 1280x1024 you will probably be looking at a pixel pitch of around 0.25-28.
A 1680x1050 22" LCD monitor would have a a pixel pitch of around 0.285 a 20" lcd would have a pixel pitch of 0.258 the difference is quite noticeable.
However the size of the pixel alone isnt the only thing that hurts lcds when it comes to jaggies, the shape and definition of the pixel doesnt help them either a LCD has inherently square edged pixels exagerating further the jaggies, CRT's pixels are made up of groups of rounded of triangles. A group of these triads make up a single "pixel" the number of triads per "pixel" is varied depending on the resolution the screen is running at. Due to this and the way phosphor dots glow very differently to the pixels in a lcd screen gives objects a smoother edge naturally.
However if you're running at 1280x1024 you are going to be looking at fairly large pixels already, apparantly without bothering you too much. Its a matter of taste really I hate jagged edges in games, really really detest it. so a 22" lcd screen would be a disaster, 20" lcd is barely tolerable, but thats just me.
I should probably mention that the "triad" pixel shape on CRT screens is referring to shadow mask screens whilst the size and phosphur glow are still relevant to aperture grille "trinitron" screens the pixel shape is more quadrilateral but still rounded edges and phosphor glow affected.
If ever you look up close at a lcd screen (a jewlers loupe is great for this) you can really see how hard edged the physical pixels are - great for text less good for natural looking scenery!