EDIT 2: Looking back at the shipping costs, batuchka's build total shipping costs is $14, which puts the total at $296 (with the PSU promo code). So if you pick up the Corsair sitcks, the total is $286.
I'd suggest trying to throw in the HD 5450, which is perfect for HTPCs and is only $25 after rebate and shipping. The older onboard video chipsets might struggle with 1080p videos. Given the extra $14 in the budget from the PSU promo and RAM, it would only take switching the motherboard in that MC combo. I'd probably pick either the Asus M4A78LT-M LE or Gigabyte GA-MA78LMT-S2. The Asus would put you right at the budget, while the Gigabyte would put you $10 under.
If you don't want to count on the rebate from the RAM, you might need to make another change besides or in addition to the board. If you're sure that the laptop drive can be used (make sure it's a SATA drive), and you really don't mide using it for a while, you could easily do that. I wouldn't recommend it though. The other option would be to drop to a single 2 GB stick of RAM, like this Patriot 2 GB stick. That would save you $11 over Corsair's after rebate price or $21 over batuchka's 2x2 GB kit's price.
EDIT 3: Found another cheaper case option: Rosewill R102-P-BK $35 after shipping. It's only a MicroATX case, so you could only get it if you switched the board to one of the ones I suggested, which would help make the HD 5450 more affordable.
I'd suggest trying to throw in the HD 5450, which is perfect for HTPCs. The older onboard video chipsets might struggle with 1080p videos.
The HD 5450 is GREAT GPU for an HTPC as MadAdmiral states. With that said... the onboard HD 4250 video chipset on the MSI 880g E-45 will do smooth blu-ray playback and bitstream HDMI audio (as will HD 4200 and HD 4290). I would suggest not buying the HD 5450 if you are budget constrained. ...can always add it later for $25.