Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

The S&P 500 (^GSPC -0.22%) rose to a brand new all-time record high this week, finishing just shy of the 1,800 milestone and reflecting the continued optimism among investors that the stock market can continue its bull-market rally into a sixth calendar year in 2014. But even with gains of more than 1.5% for the index, several key stocks made much bigger moves, with some climbing with the S&P while others dropped despite its gains. Let's look more closely at four key stocks -- J.C. Penney (JCPN.Q), Marathon Petroleum (MPC -3.67%), Denbury Resources (DNR), and Cisco Systems (CSCO 0.67%) -- to see how they affected the S&P 500 this week.

J.C. Penney jumped almost 10% this week, with investors starting to buy into the ailing retailer's turnaround story. Early in the week, J.C. Penney said that same-store sales rose in October by 0.9%, fueled in large part from a huge rise in online sales and suggesting that the retail chain's long struggle to stop bleeding customers to rivals could finally be gaining traction. Yet strong results from Macy's this week support the view that customers who fled J.C. Penney might well have liked Macy's and the other alternatives they chose. Even if revenue at J.C. Penney has hit bottom, there's no guarantee that those customers will be coming back, and that's exactly what the retailer needs to see as it enters the critical holiday season.

Marathon Petroleum posted a gain of 10% as well, with the refining giant benefiting from the reemergence of wider spreads between prices of domestic West Texas Intermediate crude oil and corresponding prices of international Brent crude. Because the price of refined products tends to track international oil prices, cheaper WTI essentially means that Marathon and its fellow refiners can get the crude oil supplies they need at discounted prices, keeping the extra proceeds as pure profit. Spreads had deteriorated to nearly zero during the summer months, but a substantial drop in WTI prices has reopened that spread to about $15 per barrel. If it stays wide, Marathon could see profits continue for a while.

Denbury Resources, on the other hand, lost almost 9% after disappointing investors who had hoped to see the energy company restructure its operations. Yet after considering plans to convert to a master limited partnership, which would have been a tax-efficient way to start returning capital to investors, Denbury decided instead to remain in its current corporate form. With so many of its rivals having done MLP conversions, Denbury plunged on that news, yet by choosing not to reorganize at this time, Denbury retains its ability to grow all of its internal operations organically and potentially produce larger profits down the road.

Cisco Systems weighed down the S&P 500 with an 8.4% decline, with its impact magnified by the tech giant's huge market capitalization. Weak revenue was the big concern, with Cisco posting less than 2% growth in sales for the quarter. Of greater concern, though, was Cisco's pronouncement that it expects a huge drop of 9% in revenue for the current quarter. With customers making disappointing decisions not to buy their typical mix of products, Cisco has to worry that competitors are taking away its business. To rebound, the tech giant needs to refocus its attention on core growth areas and make sure to close on key sales to keep its revenue up as much as possible.