The most powerful fan less video card you can buy within your budget is the SAPPHIRE Ultimate 11202-03-40G Radeon HD 7750 for $125. It can play Crysis (but not too well if you max out every single graphic setting) and it is at least equal to the HD 4850; likely a little more powerful. The video card is relatively short, but it is taller than most cards due to the large heat sink. With a maximum power consumption of 43w it consumes 1/3 the power of a HD 4850.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102980
Going down in performance, there the SAPPHIRE Ultimate 100326UL Radeon HD 6670 with 1GB of DDR5 RAM
for $85 after rebate.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102950
Another notch down is the HIS iSilence 4 H667PS1G Radeon HD 6670 with 1GB of DDR3 RAM
which decreases performance by 6% - 10% compared to DDR5 RAM. But it's cheaper at $70 after rebate.http://www.newegg.com/Product/Product.aspx?Item=N82E16814161392
Going further down there's the HIS H657H1G Radeon HD 6570 1GB 128-bit DDR3 for $60 after rebate. It performs very close to a Radeon HD 5670.http://www.newegg.com/Product/Product.aspx?Item=N82E16814161374
I would recommend the SAPPHIRE Ultimate 100326UL Radeon HD 6670 with 1GB of DDR5 RAM for your HTPC if you are only looking to play the occasional non intensive game.