Well first off the hard drive does not come with cables but you get all the cables you need with your motherboard. As far as installation goes it will no different for any other HD you buy as they are all basically the same. Seagate make one of the best quality drives about.
You can always look at this drive
Seagate Barracuda 7200.10 ST3320620AS (Perpendicular Recording Technology) 320GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive - OEM $80
http://www.newegg.com/Product/Produc...82E16822148140
As for the motherboard if you are not happy with that you can look towards a gigabyte board. I personally use the P5K-E and have not had any trouble what so ever. It could be a case of people using the original bios, not sure.
GIGABYTE GA-EP35-DS4 LGA 775 Intel P35 ATX Dynamic Energy Saver Ultra Durable II Intel Motherboard - Retail $170
http://www.newegg.com/Product/Produc...82E16813128087
from a quick search i do not think you will have any issues fitting the 9800GTX into the Antec 900 case. It is a great case and well built. The GTX is a big card and with any case you buy you may have to plan out your installation as these cards need room, so making sure you don't have hard drives in the way or cables ect.
As for the 24pin connector on the PSU, you will find that just about every board these days is a 24 pin
Honestly if you want performance AMD have slipped off the mark, google the bench marks, I don't get no prizes for recommending Intel over AMD, I just look for what performs, it your money so spend it on performance not brand loyalty