Irql_not_less_or_equal bsod

I just recently purchased a new Dell XPS 8100 (i5 650, 6GB RAM, Nvidia Geforce GTS240) and over the past week or so have been getting BSOD errors quite regularly. There doesn't seem to be any single issue - most often it happens when playing a game, but sometimes it'll happen when browsing a forum and once when I was watching a video on youtube.

I tried reverting to earlier restore points and the last known good config but to no avail. I also updated the drivers for my Nvidia card, but no luck there either. I'm at a loss for what else could be causing this, so any help would be greatly appreciated.
Minidump is attached.

it could quite possibly be the graphics card - before buying the alienware i looked at this very system - the problem is the gts240 in this pc is what is known as a tubo-cache card , this being it takes its video memory from the ram (shared memory) they are not ususaly as good or reliable as the dedicated video versions - this could explane why you get problems with the gaming side of things at leased & also could explane why you get problems with playing you-tube vids etc ( i had a ati card recently when i played any kind of flash video ie you tube etc i would get a lockup , black screen then i would have to do a hard restart) so maybe this is a similar issue - you could upgrade to a dedicated video card but when i looked in the case of this system i noticed a distinct lack of space for a decent sized video card due to some pci card installed directly below the pci express slot thus stopping you from installing any kind of double or "sandwich" desighn of card - a normal gts240 ( which i got in my alienware is a single slot card witch is about 4" wide & roughly 8" long (standard size) if you look at your gts240 you will notice that it is tiny - this is because its known as a discrete card (turbo-cache) & there is hardly any room for a bigger card - its why i didnt go for it in the end - im not saying that the issue is definately to do with the card - but in my experience turbo-cache or discrete cards are not very good - especially for gaming - i would recommend a ati radeon hd-4850 - which i replaced my gts240 with (gts 240 is a little faster on paper - but i found better results in general with the hd-4850) but from what saw of it there isnt enough room for it in there as the hd-4850 is a dual slot or "sandwitch" desighn. - maybe one of the smaller nvidia desighns might fit but i couldnt advise in this as i switched from nvidia to ati a long time ago - so in general im not farmiliar with nvidia anymore

The thing it it doesn't happen every time I try to watch a flash video - in fact it's only happened once or maybe twice, while I watch a lot of videos each day. Same thing with the gaming - It only seems to happen regularly when playing a demo for Just Cause 2 (which I've now uninstalled) and once when playing Oblivion, though I've played a lot of Oblivion over the weekend and it hasn't happened while playing it again.
I've also been playing Stalker: Call of Pripyat and I don't think I've had any errors with that either. Maybe those games are just less demanding on the video card than JC2

I'd like to avoid upgrading the video card if possible, since I only just bought this computer a couple of weeks ago and (apart from the blue screens) it seems to be able to run everything I throw at on max settings with no problem.

if its just with just cause 2 demo then quite possibly this could be due to the fact that its a demo & not a final release of the game & could have some bugs still in it & as for oblivion i wouldnt worry about this as oblivion has so many issues with it - i would not be looking at the pc - no matter what rig i have had i have never been able to run it at max framerates / resolution properly - in general i find anything with an oblivion engine (fallout 3 - etc) is generally a very unstable engine at the best of times - just wondering when you say everything at max settings - what games are these & what exact resolution & framerates are you achieving with these games - im just curious as i still have this card in my drawer & am interested in what its like as you have said maxed out - im particularly interested in what games & what framerates & resolutions you are achieving - i would appreciate the info if you have the time

ps - have you been in the case yet ? - if so just curious what is the maximum wattage output of the power supply?