Better Case, or Better HSF?

[Kaos Kid]

I would like to improve the cooling on my upstairs utility box. Specs are as follows:

Antec Sonata 1 Case
GA-MA785GMT-UD2H mATX mobo
Phenom II x 2 555BE unlocked to 4 cores
4GB Corsair DDR3
Stock Phenom cooler
Using onboard video

This is the original Sonata case that was made for low noise so has no front fan and no side fan openings it is not the best for keeping a multi-cored cpu cool. It is in my great room which is subject to temp fluctuations in the summer (kids running in and out all day). I would like to keep the cost at $50 or less (less is better, lol), which means I could probably get either a cheap low-end mATX case or a decent HSF, but not both. If I keep the case then the HSF needs to be a mid-to-low profile version. If I stay with stock cooling then the new case needs to be no more than 18" deep max to be able to close the cubby door that it is in when not in use (wife's necessity). Case height or width is not a factor, only depth. Hot air vents out of a hole I cut in the back of the cubby so the confined space is a factor but not the entire problem.
The question is which would give me more bang for the buck, the new case or the new HSF? I definitely would consider a used case or a used HSF so if anyone from the local St Louis region has something that would work PM me otherwise any suggestions you can give on this thread would be very helpful. As always, thanks for your input.

[Sttm]

Newegg has an Antec 300 Illusion which comes with 2 extra front fans over the regular case for $55.

Tiger Direct has the HAF 912 for $49, but that has less fans and is an inch over your depth requirement. Tiger Direct also has the Corsair A50 air cooler for $25, which I just bought myself and does really well for the cost.

I'd buy the Corsair A50, spend the $25 and see how that does.

Then if I had to buy a new case too, id get the 300 illusion since I already have one and its worked stellar for me and it has twice as many fans as the HAF 912 for $6 more.

[Major_A]

My opinion is the more bang for the buck is a new case. It helps cool every component more and generally has new features that weren't around when the Sonata came out.

But my question is why do you think you need either. Have programs or your computer become unstable because of heat? I have a Phenom X3 sitting outside running 24/7 in the Texas heat. Sure it gets hotter during the summer but I haven't seen any stability problems because of the added heat.

[Kaos Kid]

Because I'm seeing 53-54C when running Prime95 and ambient temps are 24C. Ambients in my Living room are much higher as summer rolls on.