You should be able to add a new video card, according o the e-machines website, your on board card has 128MB shared RAM. You should be able to disable the on board card and install a faster adapter. Your motherboard should have one open PCI-E slot.
I would add more RAM as well. According to e-machine, you can go up to 2GB, See below from E-machines web:
Operating System: Genuine Windows Vista® Home Basic1
CPU: Intel® Pentium® 4 Processor 6312
Hyper-Threading and Intel® EM64T Technology
Operates at 3.00GHz | 2MB L2 cache | 800MHz FSB
Chipset: ATI Radeon® Xpress 200
Memory: 512MB DDR2 (1 × 512MB), 533MHz (PC4200)
Expandable to 2GB
2 DDR2 Slots (Total) | 1 DDR2 Slot (Available)
Hard Drive: 160GB PATA (7200rpm)3
Optical Drive: DVD±RW 16x Multiformat Dual-Layer Optical Drive
Up to 8.5GB with Dual-Layer Media
Write max: 16x DVD±R, 6x DVD-RW, 8x DVD+RW, 4x DVD+R DL, 40x CD-R, 24x CD-RW disks
Read max: 16x DVD-ROM, 40x CD-ROM disks
Media Reader: High-Performance 15-in-1 Digital Media Manager™
Up to 40x faster
xD-Picture Card™, CompactFlash® I (CF), CompactFlash® II, Secure Digital™ (SD), Mini Secure Digital™ (Mini SD)4, Multi Media Card (MMC), Reduced Size MMC (RS-MMC)4, MMC Mobile4, MMC Plus, Memory Stick (MS), Memory Stick Duo5, Memory Stick Pro®, Memory Stick Pro® Duo5, SmartMedia, IBM Microdrive
Video: ATI Radeon® Xpress 200 Integrated Graphics
Up to 128MB of shared video memory
PCI-Express® (PCI-E x16) slot available Sound: 6-channel (5.1) high-definition audio
Amplified Stereo Speakers (USB Powered)
Communications: 56K ITU V.92-ready fax/modem (RJ-11 port)
10/100Mbps Ethernet LAN (RJ-4