Now you've picked out the frame and groupset that's perfect for you and your budget. However, you're not done yet. Don't forget to budget in some of the items that aren't included in the bike, but are essential for happy cycling.
First and foremost on your list should be a helmet. I am a helmet advocate. Do helmets conclusively protect cyclists? Some studies say yes, others no. However, to me any piece of padding you can include between yourself and the hard ground is well worth it. The good thing about bicycle helmets is they are all certified by the same CPSC standard in the US. So in theory a $20 helmet should protect you as well as $200 helmet. If you're looking for a way to save a little cash, this is an item where a less expensive model can be used. What separates more expensive helmets from cheaper ones are lighter weights and better ventilation. It's a good idea to take these into account, as comfort can be very important during long rides in hot climates. The Specialized Echelon is a great helmet for entry level riders. It has the same shape and ventilation system as the $200 S-Works, is lightweight, has a great adjustment system, and costs just $60.
The Specialized Echelon is a great choice for a first helmet
The next item on your purchase list should be pedals and shoes. Your new bike will likely come with plastic pedals equipped with toe clips. These are fine on short rides. However on longer rides they can lead to numb toes and don't promote an efficient pedal stroke. "Clipless pedals," so named because they lack toe clips, are the ones you actually clip into. Since your feet are connected to the pedal you can not only push down, but also pull up on creating a more efficient pedal stroke as well as recruiting the hamstrings to share some of the burden. Stiff soled cycling shoes help prevent numb toes and ensure that all pushing or pulling energy your feet exert help to move the crank. Pedals and shoes are the one accessory that will actually make you faster. Shimano's 105 pedal is a great entry level road pedal. They are easy to clip and out of and the wide base gives a nice solid platform to prevent hot spots. Although the cleat might need to be replaced every season, it doesn't require the maintenance Speedplay cleats do. A careful shopper can find them for $60-80 at various online outlets. Shoes are an item of great personal preference, so I am hesitant to recommend a brand or model. Any stiff soled shoe will do as long as it fits and is comfortable. This is one item you should try before you buy. Most cycling shoes use Euro sizing, so the size of your tennis shoes will probably be different from your cycling shoes.
Shimano's 105 Pedals are a great entry level option
While a Camelbak might be perfectly appropriate for mountain biking, it's faux pas on a road bike. Water bottles and cages are a must. Rides can last for hours so it's important to keep your body well hydrated along the way. Fortunately this doesn't have to be anything fancy. Unless you want to shave every ounce off your bike, skip the fancy carbon cages and stick with aluminum ones. Throw in a couple of cheap water bottles and your setup is complete. A savvy negotiator should be able to negotiate these in as freebies when they purchase their bikes. If you can't, this shouldn't cost more than $15.
Along the same lines, while cycling you're bound to have a flat someday and you should be prepared to fix them yourself. Any cyclist worth his salt has a seat bag filled with a spare tube, patch kit, and some sort of inflation system (either CO2, a hand pump, or both). A portable multi-tool is also a good idea in case a bolt may need tightening along the way.
A proper pair of cycling shorts is another item I wouldn't leave the store without. Cycling shorts like helmets are another item which can get very expensive. However, this is an area where I wouldn't substitute for quality. A good pair of cycling shorts has a comfortable chamois which takes pressure off your sit bones. A quality lycra that compresses your leg muscles can help stave off fatigue. Bib shorts offer the added benefit of not having a waist band that digs into your side and the shoulder straps hold the chamois in place better than standard shorts.
All told, these items can add up. However, now is the season to buy since deep discounts can be had as 2009 models are closed out. Also, there are always deals on websites like Bonktown and Chain Love which offer 50-80% off items. Other companies like Competitive Cyclist offer deals only announced on their twitter feeds. The accessories you choose can contribute to a pleasant cycling experience just as much as a bike, so careful selection is a worthwhile investment. If you can't stand the numbness in your toes or the pain in your butt, the best bike in the world won't solve those problems.