My advice is worthless - I am an unqualified muppet...
A combi bolier is relatively easy to install and cheap to run - good for small properties and gives instant hot water - not so good for a big house - a bit more complex so will be prone to failure a litle more often - needs no hot water storage tank so you will not have the advantage of back-up immersion heater for hot water or an airing cupboard.
A regular condensing boiler will have a hot water tank and you can have an immersion heater.
A system boiler comes with valves and pump built in and is more costly but easier to install - so they say.
All modern boilers have about 90% or better effiency - your old boiler may be as low as 65%.
Expect to pay between £2.5k to £5k for your new installation - British Gas tend to be more expensive and use pressure selling tactics from what I can tell on here.
I believe - but am open to correction because I'm an unqualified muppet - that new installations need to have a 22mm gas supply pipe rather than 15mm - this may mean extra cost for any new installation.
I have read trade forums where the experts argue about the merits of one boiler over another - many seem to think Worcester Bosh quite good.
A UK government scheme allows I think £400 boiler scrappage refund for inefficient below 70% but it's a limited budget on a first come basis.
Check the efficiency of your existing boiler here...
http://www.homeheatingguide.co.uk/efficiency-tables.php
Good luck...
Peccavi, December 2009
