The motherboard in that PC will either be ATX, micro-ATX or mini-ITX, it won't be custom built for the case at all (if it is, that's the first I heard of it). I'm guessing it's not mini-ITX, too. At any rate, the vast majority of modern PC tower cases will support all three types. You can pick any case you like, its down to preference and budget. If you're shooting for a gaming system, then look at Corsair or Silverstone for a start.
The case itself has little bearing on your system being used for gaming, other than considering good airflow to get heat out. Also make sure the case has filters, which is always a plus. Your system is S1155, which is good, as it is upgradeable to pretty much anything you want.
I would check the motherboard model to confirm it has PCIe 2.0, and look at getting a dedicated graphics card. It currently uses the CPU onboard graphics, which are not good for anything more than the most basic of games (or older titles). You'll also need to seriously upgrade the PSU...300W isn't very much, and I'm betting it'll be a generic PSU brand you have right now. Depending on what GPU you go for, you'll want a minimum of 500-600W, from a manufacturer like Corsair, Silverstone, Enermax, Be Quiet! or Seasonic, to name a few.
I meant to say open up your case and take a look at what you have, then Google both ATX and micro-ATX to see what you have, but CPU-Z will give you your model number. If you've installed a 440, then you have a PCIe x16 slot (which is the size) but it would be worth confirming if the model has PCIe 2.0 also. Odds are it does, though.
To check if the board fits you need to check if the holes to mount it are standard. The size will not be custom but some OEMS have in the past/do put the holes in different places so they are not compatible with standard cases (Dell springs to mind)
How many ram slots does it have? As far as I know matx and mini-itx only have two ram slots whereas atx motherboards have 4/6 slots. Considering its got relatively modern hardware in it, it should have 4 slots if it was an atx motherboard.
I agree with diellur, you will need a graphics card if you plan on playing something other than pinball xD In terms of the PSU, you could get away with 500W if you get a lower power consuming graphics card (stay away from the gtx480 or gtx470 - they guzzle the watts) and don't add a load of harddrives.
For the case, if it is an atx then go for a mid tower or full tower case - I would advise a full tower as you probably will end up upgrading it or just adding stuff to it and it does get pretty cramped in a mid tower.
Oh and also on the PSU, check the power connectors which the power supply comes with to make sure that you have enough connectors to power everything. I'm glad I checked before buying my graphics card otherwise I would have been in for quite a shock.
Full tower cases are compatible with matx, micro-itx and atx, it just means you use less motherboard standoffs. Considering its only a micro atx you will be fine with just a mid tower (mid towers cost less aswell). Mid towers can range from £10+ but full towers will range from £60/70+ I would only get a full tower if: you have an atx motherboard, you plan on getting a long graphics card, you get a graphics card but want to slap an Arctic cooling accelero xtreme plus II aftermarket cooler on it or you plan on doing watercooling in the future.
Some nice budget mid towers include:
- Corsair Carbide 300R
- Corsair Carbide 400R
- CM 690 II advanced
- CM Scout
- NZXT Phantom 410
- Fractal design R3
- Antec 900
If you want to have an sli config go for a full tower, you'll have more space and you'll need a motherboard with two 16x pci express slots. You could get a mid tower but you'd need to put some decent fans in it - if you by two non-reference gtx460 's (Gigabyte is good) you would be fine keeping them cool. The problem with non-reference cards is that they are near impossible to water cool with a full watercooling block as the screw placements differ from the reference cards.
If you want to watercool your two gtx460s, get a full tower as you will have more space for the pipes, radiator, pump, reservoir etc.
If you are going to get 2 reference gtx460s, get a full tower as you can pack it full of fans to keep the cards cool.
If you are going to get 2 non-reference gtx460's, they will run cooler so you won't need as many case fans to cool them down, get a mid tower.
Having said this, some non-reference cards have very tall coolers on them, so if you did chose non-reference over reference, check how many pci slots they take up and make sure you will have enough pci slots for all of your hardware, graphics cards, wifi card etc. If you don't have enough, get a full tower, it will have more pci slots.
In terms of cases:
- The nzxt phantom is *very* plastic, so don't get if you think it looks cheap, but it does have a lot of space, it has good fan filters and a lot of space.
- Antec cases are very good at cooling.
- Fractal design focus less on good cooling and more on making your pc silent. Also inside the door, the front looks a bit cheap. Having said that, they are very classy cases, have good build quality and good dust filters.
- The corsair 400R can cream a coolermaster haf at temperatures - Corsair also have very good build quality and they really keep the temps down.
- The gelid darkforce isn't particularly popular, but it is a good case and more refined version of coolermaster's cm 690 II advanced. (it also has a nice window)
- The Nzxt switch 810 is very good all round full tower. It has good airflow, 10 fans!!
In a temp test (the temperature taken is the cpu temperature) it stays cooler on load than a corsair 600t, NZXT h2 and a corsair 650d. http://www.overclockersclub.com/reviews/nzxt_switch810/... This has some pretty good comparisons.
To chose the right case I would just go on a website like scan or ebuyer or newegg if you live in US and just read people's reviews to get a real insight into what the case is like from the people who own it.