Buy a bike that's actually suited to your needs. I see so many people sweating buckets with their mountain bike and 3" wide hyper-grip tyres when they only ever go on the road.
You only need rear suspension if you're going to be going off road. If it's just for cycling about, go for a hybrid,which is a lightweight frame with higher gears for road use and nice, slick road tyres, but with a more comfy saddle and straighter 'mountain bike' handlebars instead of the ram-horn race bike style.
Disk brakes aren't really any use unless you're going to be going offroad in heavy mud, etc. Regular 'V-' brakes will be more than ample for road use and will be plenty enough to stop you.
What to spend: it depends. For more money you get better gearing (changes quicker & cleaner, stays true for a much longer time before needing adjustment, causes much less frustration), you also get, hopefully, a lighter frame and (if you're going for it) better quality suspension. I wanted to spend a fair bit on a bike (£350), but wanted a high-end regular bike, rather than a low end pro bike, if that makes sense. Here's what I looked for:
v-Brakes (I didn't like the idea of disks at the time)
Metal pedals (plastic ones are slippy and I just don't like them)
Decent gearing
Mountain bike frame, as I wanted to take it offroad occasionally.
A 'hard tail (no rear suspension) but fairly decent front forks that were adjustable (road/offroad usage).
Decent wheels (I hate cheap ones that buckle all the freaking time)
I got the GT avalanche bike which was awesome, with all the above features. I spent £30 and put the slickest tyres on it possible (makes the thing fly), but I can always swap out my offroad tyres if I fancy getting muddy or whatever.
The only issue I had is that being a mountain bike, the gearing is low such that, with a tail wind or downhill, I 'spin out (can turn the pedals quick enough) at about 27mph. To solve this, I put on a huge top gear at the front which was a real PITA to fit (needed to adjust front derraileur and add a couple of links to the chain, which means on the smallest front and smallest rear gears, the chain hangs kinda slack) but now I can hit mid to high 30's which is sweet.
Essentially:aim for less features (no full suspension) but have a read up and make sure that what it *does* come with is higher quality. Would you rather a fully featured bike that lasts 4 months after which your rims will be all bent, the gearing knackered -like a 20 second chugging noise before it changes, etc- and generally more weight, or a more basic bike (maybe don't even go for front suspension) that'll actually last and not frustrate you when you ride it?