having lots of cards is great and opening a booster box is even better but over the years (im in my 20's) i've had many, many booster boxes and full sets of sets twice over sometimes but in the end constructing a solid deck is not about how many cards you have its more about what cards you have. you must only buy singles, its the only way! paying for each card you need in the end is alot cheaper then buying tons of packs because the tax alone is alot after awhile if your in the US. build, test and play a deck many times before purchasing the cards though because you might find that you spent money on something that wont be in the final deck list, so be careful buying cards too soon, i know i've done it and kicked my self when $30+ of cards i cut from the deck list.
buy a booster box every now and again just for trading purposes because buying some single is not worth it and trading is much better to get the more expensive cards!!!
hope this helps... i mean i have every card i want/need with only spending a small amount of cash in the long run... the link below is for a family member of mine that sells pokemon cards, it is his main job that he take very seriously, everything is perfect and i would go nowhere else when it comes to card buying!!!
HARAY FOR!!!!! www.Professor-oak.com
GOOD LUCK AND HAPPY PLAYING