the biggest problem with trying to utilize OEM cases (such as Dell, HP etc) is that each case is specifically tayored to hold the specific Power unit, motherboard, drives, cards etc. that are in there by default. For example, the motherboard from your 720 probably wouldn't fit in many other cases, because BTX is an all-but discarded form factor. BTX never really caught on, so i'll be damned if you can find a BTX motherboard in the spec you want. I recommend looking at Cooler Master cases, and perhaps Lian Li or antec - since many of them hold the kooky 'gamer' looks, specifically say the windowed Lian Li cases or the CM HAF X. The case is very important for several reasons.
1. Cooling - the better the airflow in the case, the better the cooling will be. Also, certain cases will require mods to hold liquid cooling radiators/blocks, whereas others have facilities for Liquid cooling by standard. Also, people who use beefy CPUs usually prefer a Beefy cooler - obviously if the cooler is taller than the case is wide then it won't fit.
2. Form factor - a Micro-ATX case won't hold an ATX Board (like trying to put a big stone in a small gap) but the majority of ATX cases will hold ATX boards.
3. Space - If the case if to small, and you pick a GPU with a very long board, then you're going to have problems fitting it in. Say for example, the Gigabyte HD 6950.
Right now, the most popular enthusiast CPUs for builds are definitely Intel's new 1155 Sandy Bridge chips. They don't cost a whole lot more than the 1st generation Core series, and run cooler and faster. You can pick up an i5 2500k for around £160 (i notice you're in the UK, so this is pretty easy for me to recommend parts to you)
Scan.co.uk: Intel CPU Core i5 2500K Unlocked Sandy Bridge Quad Core Processor - BX80623I52500K
This is one of the best chips i've come across for new builders - It's dead simple to install and presents very few problems now that Intel have sorted out the chipset issues.
Back on the subject of cases, you could look at NZXT - they almost embody that insane design we like so much
Scan.co.uk: NZXT M59 Black Mid Tower Computer Case
As for motherboards, since i recommended the 2500k, i also recommend this 1155 motherboard.
Scan.co.uk: Gigabyte GA-P67A-UD4 Intel P67 Express Socket 1155 Motherboard - GA-P67A-UD4/B3
it's middle-of-the-range ans good value IMHO.
As for power units to power it all, 650W of high-quality branded PSU would suffice, for example
Scan.co.uk: Lian-Li Maxima Force PS-A650GB 650W Power Supply (PSU)
High performance memory is pretty cheap in modern times, i recommend this
Scan.co.uk: Corsair Memory Vengeance 8GB DDR3 1600 MHz CAS 9 XMP Dual Channel Desktop - CMZ8GX3M2A1600C9
And obviously, the gamer's best friend - the GPU; If you want to stick with NVIDIA, go with
Scan.co.uk: EVGA GTX460 SSC Super SuperClock NVIDIA Graphics Card - 1GB - 01G-P3-1380-KR
or if you fancy a change, i recommend
Scan.co.uk: MSI HD 6870 HAWK ATI - AMD Radeon Graphics Card with Twin Frozr III fan - 1GB+ Free Shogun 2* - R6870 Hawk
Hard drives and such are dependent on what you want out of them, so i'll leave that to you. I know I sort of went off a little here, but i wouldn't like the idea of spending alot on the previous generation of i7 when the newer i5s have so much more to offer, and run cooler too.
Speaking of which, You can get a 'liquid' cooler that is entirely hassle-free, but it's only for your CPU, and it's a bit of a gimmick, but it certainly does the job.
Scan.co.uk: Corsair H70 Hydro Series CPU Cooler - CWCH70
^_^