how loaded is the server now? compressing will increase the server load, but not extreme!
this is hard to predict, you will need to analyse the load and see if thats get you into trouble...
if then php will be the limit or the db, there are caching tools (like xcache) what will help you out on this

for example this page while i type has 84 HTTP requests, with the mod_expire this would be reduced to around 5!
even when the stuff is cached currently, there is still a HTTP request to check the cache...
as w3c said only 2 requests per domain are paralell, that means the user will make a crazy 40 requests in series...

and your server must handle all this useless requests! making load for nothing...
to test it out, take ff, install (in that order):
https://addons.mozilla.org/de/firefox/addon/firebug/https://addons.mozilla.org/de/firefox/addon/yslow/http://code.google.com/intl/de-DE/speed/page-speed/download.htmlthis tools will show you the pageload times, tranfered volumes, HTTP requests and much more.
get your server stats (get into ssh, "top"), look at the loads and then insert the htaccess and watch what happens...
check on the site if it works like expected, [STRG]+[F5] and look into Yslow "Statistics" there must be a significant drop in requests and page size.
if your cpu gets overloaded take the htaccess down!
i can only tell you that on my biggest forum (record ~ 10k hits per second) on a crapy shared server handled it like a charm with this htaccess (and some other things)

for detailed things, pn me, for details i need your hardware and software config and some load figures...
lets make this page fly
