Updated from 1:13 p.m. EDT

Shares of

Ptek Holdings

(PTEK)

were among the worst-performing technology stocks Tuesday after the company warned in a

Securities and Exchange Commission

filing that its Premiere Conferencing unit could lose some of its

IBM

(IBM) - Get Report

business next year.

The conferencing and messaging company said that Premiere's agreement with IBM expires on Dec. 31, 2004, and that there is no assurance that it will be able to retain IBM as a customer. What's more, the IBM relationship does not include any revenue commitments or termination charges. According to Ptek, "IBM is actively exploring the use of new technologies, such as voice over Internet Protocol (or VoIP), to help lower its costs of conferencing." A shift to VoIP, whether it's provided by Premiere or not, would "result in a decrease in revenue to Premiere Conferencing for this account over time," the company said. IBM accounted for about 11.3% of sales during the first half of 2004. Shares of Ptek Holdings traded down $2.57, or 24.4%, to $7.96.

Taser

(TASR)

rose after

the company said it received one new order and three follow-on orders totaling more than $724,000. The follow-on orders came from the police department in Glendale, Ariz., which ordered 196 weapons; the Pinellas County, Fla., Sheriff's department, which ordered 176 units; and an unnamed law enforcement agency that ordered $247,000 worth of stun guns. The Fremont, Calif. police department placed an initial order for 169 weapons. All of the orders are expected to ship during the third quarter. Shares of Taser traded up $1.48, or 5.6%, to $28.11.

Shares of

Nvidia

(NVDA) - Get Report

rose after the company's board approved

a $300 million stock buyback. The chipmaker said that it would fund the stock purchases from available working capital. As of June 30, it had over $600 million in cash, cash equivalents and marketable securities. "We believe that the repurchase program is a good investment of available funds and underscores our commitment to enhancing shareholder value by assisting in offsetting dilution from employee stock plans," the company said. Shares of Nvidia traded up 78 cents, or 7.9%, to $10.63.

Sonic Solutions

(SNIC)

rose after it agreed to purchase

Roxio's

(ROXI)

CD and DVD software unit for $80 million. Sonic will pay Roxio $70 million in cash and $10 million in stock. The acquisition is expected to close during the fourth quarter. Shares of Sonic Solutions traded up $1.69, or 13.4%, to $14.32.

Shares of

Dobson Communications

(DCEL)

fell after the company posted weaker-than-expected-second-quarter earnings and lowered its subscriber outlook for the full year. The wireless phone services company posted a second-quarter loss of 12 cents a share on sales of $252.4 million. Analysts were expecting a loss of 10 cents a share. Looking ahead, the company said that it would end the year with 386,500 gross subscriber additions. Previously, it said that it would add between 444,000 and 455,000 subscribers. Full-year EBITDA will now be in the range of $335 million to $345 million, down from its previous guidance of $390 million to $425 million. Shares of Dobson Communications traded down $1.30, or 54.2%, to $1.10.

Other technology volume leaders included

Cisco

(CSCO) - Get Report

, up 41 cents to $20.46;

Microsoft

(MSFT) - Get Report

, up 54 cents to $27.72;

Intel

(INTC) - Get Report

, down 3 cents to $22.54;

Oracle

(ORCL) - Get Report

, up 23 cents to $10.60; and

Lucent

(LU)

, up 2 cents to $3.01.