gsmarena.com has detailed info on pretty much every phone.
As for PAYT, if you know roughly what proportion of the charges are for voice, I can almost guarantee you'd save by switching tto Asda PAYG, SIM pack is 50p I think (but free if you ask for a top-up voucher at the same time. You could get a new SIM pack each time you buy a top-up if you want to look for an easily memorable number, unless you plan to move your Vodafone number on to Asda} and they charge 4p/text and 8p/min to 01/02/03 landline numbers or mobiles... any time of day, no peak and off-peak... Quite a lot better than some which have 20p for same network, 50p for calls to other networks or first 5 minutes are 15p/min then next 5 minutes are free, blah, blah, blah...
Oh, and it doesn't have a 30 day expiry like some top-ups on some networks... so no prob if you top up before a 2 week holiday and come back with 20 quid still left in credit on the phone... Asda also do various phones which are unlocked, so can work with any network. Mostly they don't require a top-up when you buy a phone, unlike some shops (Argos used to sell just the phones, locked to different networks, but now it seems they are forced to charge for a top-up as well).
As for comparing prices, I think I'd suggest grabbing one of the monthly 'magazines' from a branch of Carphone Warehouse... they do a summary of most of the phone facilities, so you can spot a few you might like, and then the back page shows the pricing for them on PAYG with each of the networks.
Most require a 10 pound top-up, sometimes 20 pounds top-up. If you spot what you want on Vodafone, you could use an Asda SIM anyway. If you check in Asda Living, they may have an unlocked model or one already on Vodafone.
You may also want to look for another phone that uses the M2 type data card that most Sony Ericssons use, or you could flog the one you have for a few quid on Ebay or with the old phone, to cover for a new memory card for whatever phone you decide on.
Finally, you might find some bargains if you check on e2save.com (also run by CPW) or some of the other online stores, and try yoodoo.co.uk for user reviews, and look on play.com and perhaps even Froogle (Google's shopping site) if you know model numbers for phones which meet your needs.
For a list of other mobile sites, since I am out of touch with shopping/ comparison /review sites for mobiles, you might find NewsNow worth checking.
NewsNow scans around 30,000 websites every few minutes looking at the headlines, and allows searches for single keywords {offered free, multiple keywords only with a costly subscription}. Chuck in 'camera' under the Mobile Phones heading and then you'll see news of the latest phones (but more useful, perhaps, is that this gives lots of links to different review/ news websites, so you'll find sites you might never have heard of)... It carries news items from sites worldwide so look for a UK flag alongside the headlines (else you might see phones not available in the UK)...